Characterization and localization of the cytoplasmic dynein heavy chain in Aspergillus nidulans

VezA/vezatin facilitates proper assembly of the dynactin complex in vivo

Cytoplasmic dynein-mediated intracellular transport needs the multi-component dynactin complex for cargo binding and motor activation. However, the cellular factors involved in dynactin assembly remain unexplored. Here, we found in Aspergillus nidulans that the vezatin homolog VezA is important for dynactin assembly. VezA affects the microtubule plus-end accumulation of dynein before cargo binding and cargo-adapter-mediated dynein activation, two processes that both need dynactin. The dynactin complex contains multiple components, including p150, p50, and an Arp1 (actin-related protein 1) mini-filament associated with a pointed-end sub-complex. VezA physically interacts with the Arp1 mini-filament either directly or indirectly. Loss of VezA significantly decreases the amount of Arp1 pulled down with pointed-end proteins, as well as the protein levels of p50 and p150 in cell extract. Using various dynactin mutants, we further revealed that the dynactin assembly process must be highly coordinated. Together, these results shed light on dynactin assembly in vivo.

Small Sized Yet Powerful: Nuclear Distribution C Proteins in Plants

The family of Nuclear Distribution C (NudC) proteins plays a pivotal and evolutionarily conserved role in all eukaryotes. In animal systems, these proteins influence vital cellular processes like cell division, protein folding, nuclear migration and positioning, intracellular transport, and stress response. This review synthesizes past and current research on NudC family members, focusing on their growing importance in plants and intricate contributions to plant growth, development, and stress tolerance. Leveraging information from available genomic databases, we conducted a thorough characterization of NudC family members, utilizing phylogenetic analysis and assessing gene structure, motif organization, and conserved protein domains. Our spotlight on two Arabidopsis NudC genes, BOB1 and NMig1, underscores their indispensable roles in embryogenesis and postembryonic development, stress responses, and tolerance mechanisms. Emphasizing the chaperone activity of plant NudC family members, crucial for mitigating stress effects and enhancing plant resilience, we highlight their potential as valuable targets for enhancing crop performance. Moreover, the structural and functional conservation of NudC proteins across species suggests their potential applications in medical research, particularly in functions related to cell division, microtubule regulation, and associated pathways. Finally, we outline future research avenues centering on the exploration of under investigated functions of NudC proteins in plants.

Dynein activation in vivo is regulated by the nucleotide states of its AAA3 domain

Cytoplasmic dynein is activated by the dynactin complex, cargo adapters and LIS1 (Lissencephaly 1). How this process is regulated in vivo remains unclear. The dynein motor ring contains six AAA+ (ATPases associated with diverse cellular activities) domains. Here, we used the filamentous fungus Aspergillus nidulans to examine whether ATP hydrolysis at AAA3 regulates dynein activation in the context of other regulators. In fungal hyphae, early endosomes undergo dynein-mediated movement away from the microtubule plus ends near the hyphal tip. Dynein normally accumulates at the microtubule plus ends. The early endosomal adaptor Hook protein, together with dynactin, drives dynein activation to cause its relocation to the microtubule minus ends. This activation process depends on LIS1, but LIS1 tends to dissociate from dynein after its activation. In this study, we found that dynein containing a mutation-blocking ATP hydrolysis at AAA3 can undergo LIS1-independent activation, consistent with our genetic data that the same mutation suppresses the growth defect of the A. nidulans LIS1-deletion mutant. Our data also suggest that blocking AAA3 ATP hydrolysis allows dynein activation by dynactin without the early endosomal adaptor. As a consequence, dynein accumulates at microtubule minus ends whereas early endosomes stay near the plus ends. Dynein containing a mutation-blocking ATP binding at AAA3 largely depends on LIS1 for activation, but this mutation abnormally prevents LIS1 dissociation upon dynein activation. Together, our data suggest that the AAA3 ATPase cycle regulates the coordination between dynein activation and cargo binding as well as the dynamic dynein-LIS1 interaction.

LIS1 regulates cargo-adapter-mediated activation of dynein by overcoming its autoinhibition in vivo

Deficiency of the LIS1 protein causes lissencephaly, a brain developmental disorder. Although LIS1 binds the microtubule motor cytoplasmic dynein and has been linked to dynein function in many experimental systems, its mechanism of action remains unclear. Here, we revealed its function in cargo-adapter-mediated dynein activation in the model organism Aspergillus nidulans Specifically, we found that overexpressed cargo adapter HookA (Hook in A. nidulans) missing its cargo-binding domain (ΔC-HookA) causes dynein and its regulator dynactin to relocate from the microtubule plus ends to the minus ends, and this relocation requires LIS1 and its binding protein, NudE. Astonishingly, the requirement for LIS1 or NudE can be bypassed to a significant extent by mutations that prohibit dynein from forming an autoinhibited conformation in which the motor domains of the dynein dimer are held close together. Our results suggest a novel mechanism of LIS1 action that promotes the switch of dynein from the autoinhibited state to an open state to facilitate dynein activation.

NudC regulates photoreceptor disk morphogenesis and rhodopsin localization

The outer segment (OS) of rod photoreceptors consist of a highly modified primary cilium containing phototransduction machinery necessary for light detection. The delivery and organization of the phototransduction components within and along the cilium into the series of stacked, highly organized disks is critical for cell function and viability. How disks are formed within the cilium remains an area of active investigation. We have found nuclear distribution protein C (nudC), a key component of mitosis and cytokinesis during development, to be present in the inner segment region of these postmitotic cells in several species, including mouse, tree shrew, monkey, and frog. Further, we found nudC interacts with rhodopsin and the small GTPase rab11a. Here, we show through transgenic tadpole studies that nudC is integral to rod cell disk formation and photoreceptor protein localization. Finally, we demonstrate that short hairpin RNA knockdown of nudC in tadpole rod photoreceptors, which leads to the inability of rod cells to maintain their OS, is rescued through coexpression of murine nudC.-Boitet, E. R., Reish, N. J., Hubbard, M. G., Gross, A. K. NudC regulates photoreceptor disk morphogenesis and rhodopsin localization.

p25 of the dynactin complex plays a dual role in cargo binding and dynactin regulation

Cytoplasmic dynein binds its cargoes via the dynactin complex and cargo adapters, and the dynactin pointed-end protein p25 is required for dynein-dynactin binding to the early endosomal dynein adapter HookA (Hook in the fungus Aspergillus nidulans). However, it is unclear whether the HookA-dynein-dynactin interaction requires p27, another pointed-end protein forming heterodimers with p25 within vertebrate dynactin. Here, live-cell imaging and biochemical pulldown experiments revealed that although p27 is a component of the dynactin complex in A. nidulans, it is dispensable for dynein-dynactin to interact with ΔC-HookA (cytosolic HookA lacking its early endosome-binding C terminus) and is not critical for dynein-mediated early endosome transport. Using mutagenesis, imaging, and biochemical approaches, we found that several p25 regions are required for the ΔC-HookA-dynein-dynactin interaction, with the N terminus and loop1 being the most critical regions. Interestingly, p25 was also important for the microtubule (MT) plus-end accumulation of dynactin. This p25 function in dynactin localization also involved p25's N terminus and the loop1 critical for the ΔC-HookA-dynein-dynactin interaction. Given that dynactin's MT plus-end localization does not require HookA and that the kinesin-1-dependent plus-end accumulation of dynactin is unnecessary for the ΔC-HookA-dynein-dynactin interaction, our results indicate that p25 plays a dual role in cargo binding and dynactin regulation. As cargo adapters are implicated in dynein activation via binding to dynactin's pointed end to switch the conformation of p150, a major dynactin component, our results suggest p25 as a critical pointed-end protein involved in this process.

The actin capping protein in Aspergillus nidulans enhances dynein function without significantly affecting Arp1 filament assembly

The minus-end-directed microtubule motor cytoplasmic dynein requires the dynactin complex for in vivo functions. The backbone of the vertebrate dynactin complex is the Arp1 (actin-related protein 1) mini-filament whose barbed end binds to the heterodimeric actin capping protein. However, it is unclear whether the capping protein is a dynactin component in lower eukaryotic organisms, especially because it does not appear to be a component of the budding yeast dynactin complex. Here our biochemical data show that the capping protein is a component of the dynactin complex in the filamentous fungus Aspergillus nidulans. Moreover, deletion of the gene encoding capping protein alpha (capA) results in a defect in both nuclear distribution and early-endosome transport, two dynein-mediated processes. However, the defect in either process is less severe than that exhibited by a dynein heavy chain mutant or the ∆p25 mutant of dynactin. In addition, loss of capping protein does not significantly affect the assembly of the dynactin Arp1 filament or the formation of the dynein-dynactin-∆C-HookA (Hook in A. nidulans) complex. These results suggest that fungal capping protein is not important for Arp1 filament assembly but its presence is required for enhancing dynein function in vivo.

Endocytic recycling via the TGN underlies the polarized hyphal mode of life

Intracellular traffic in Aspergillus nidulans hyphae must cope with the challenges that the high rates of apical extension (1μm/min) and the long intracellular distances (>100 μm) impose. Understanding the ways in which the hyphal tip cell coordinates traffic to meet these challenges is of basic importance, but is also of considerable applied interest, as fungal invasiveness of animals and plants depends critically upon maintaining these high rates of growth. Rapid apical extension requires localization of cell-wall-modifying enzymes to hyphal tips. By combining genetic blocks in different trafficking steps with multidimensional epifluorescence microscopy and quantitative image analyses we demonstrate that polarization of the essential chitin-synthase ChsB occurs by indirect endocytic recycling, involving delivery/exocytosis to apices followed by internalization by the sub-apical endocytic collar of actin patches and subsequent trafficking to TGN cisternae, where it accumulates for ~1 min before being re-delivered to the apex by a RAB11/TRAPPII-dependent pathway. Accordingly, ChsB is stranded at the TGN by Sec7 inactivation but re-polarizes to the apical dome if the block is bypassed by a mutation in geaA^gea1 that restores growth in the absence of Sec7. That polarization is independent of RAB5, that ChsB predominates at apex-proximal cisternae, and that upon dynein impairment ChsB is stalled at the tips in an aggregated endosome indicate that endocytosed ChsB traffics to the TGN via sorting endosomes functionally located upstream of the RAB5 domain and that this step requires dynein-mediated basipetal transport. It also requires RAB6 and its effector GARP (Vps51/Vps52/Vps53/Vps54), whose composition we determined by MS/MS following affinity chromatography purification. Ablation of any GARP component diverts ChsB to vacuoles and impairs growth and morphology markedly, emphasizing the important physiological role played by this pathway that, we propose, is central to the hyphal mode of growth.

Filamentous fungi form long tubular cells, called hyphae, which grow rapidly by apical extension, enabling these sessile organisms to explore substrates and facilitating tissue invasion in the case of pathogenic species. Because the shape of the hyphae is determined by an external cell wall, hyphal growth requires that cell-wall sculpting enzymes polarize to the tips. Endocytosis is essential for hyphal growth, and it was suspected that this results from its participation in a recycling pathway that takes up cell-wall enzymes from the plasma membrane and re-delivers them to the apex. Here we track the trafficking of a chitin synthase (a cell-wall modifying enzyme) to demonstrate that it is polarized by endocytic recycling. This chitin synthase is delivered by exocytosis to the apex, but diffuses away until being captured by a subapical collar of actin patches (sites of endocytosis) from where it reaches a sorting endosome before undergoing transport to the nearest trans-Golgi cisternae and incorporating into secretory vesicles that re-deliver the enzyme to the apex. Because impairing transit across this pathway compromises apical extension markedly and results in severe morphological defects, the pathway could be manipulated to prevent fungal pathogenicity of plants and humans, an enormous burden on human welfare.

Discovery of a vezatin-like protein for dynein-mediated early endosome transport

In filamentous fungi, dynein moves early endosomes away from the hyphal tip. Aspergillus genetics is used to identify a vezatin-like protein, VezA, which is critical for dynein-mediated transport of early endosomes. VezA localizes to the hyphal tip in an actin-dependent manner and regulates the interaction between dynein and early endosomes.

Early endosomes are transported bidirectionally by cytoplasmic dynein and kinesin-3, but how the movements are regulated in vivo remains unclear. Here our forward genetic study led to the discovery of VezA, a vezatin-like protein in Aspergillus nidulans, as a factor critical for early endosome distribution. Loss of vezA causes an abnormal accumulation of early endosomes at the hyphal tip, where microtubule plus ends are located. This abnormal accumulation depends on kinesin-3 and is due to a decrease in the frequency but not the speed of dynein-mediated early endosome movement. VezA-GFP signals are enriched at the hypha tip in an actin-dependent manner but are not obviously associated with early endosomes, thus differing from the early endosome association of the cargo adapter HookA (Hook in A. nidulans). On loss of VezA, HookA associates normally with early endosomes, but the interaction between dynein-dynactin and the early-endosome-bound HookA is significantly decreased. However, VezA is not required for linking dynein-dynactin to the cytosolic ∆C-HookA, lacking the cargo-binding C-terminus. These results identify VezA as a novel regulator required for the interaction between dynein and the Hook-bound early endosomes in vivo.

Microtubule plus end-tracking proteins play critical roles in directional growth of hyphae by regulating the dynamics of cytoplasmic microtubules in Aspergillus nidulans

Cytoplasmic microtubules (MTs) serve as a rate-limiting factor for hyphal tip growth in the filamentous fungus Aspergillus nidulans. We hypothesized that this function depended on the MT plus end-tracking proteins (+TIPs) including the EB1 family protein EBA that decorated the MT plus ends undergoing polymerization. The ebAΔ mutation reduced colony growth and the mutant hyphae appeared in an undulating pattern instead of exhibiting unidirectional growth in the control. These phenotypes were enhanced by a mutation in another +TIP gene clipA. EBA was required for plus end-tracking of CLIPA, the Kinesin-7 motor KipA, and the XMAP215 homologue AlpA. In addition, cytoplasmic dynein also depended on EBA to track on most polymerizing MT plus ends, but not for its conspicuous appearance at the MT ends near the hyphal apex. The loss of EBA reduced the number of cytoplasmic MTs and prolonged dwelling times for MTs after reaching the hyphal apex. Finally, we found that colonies were formed in the absence of EBA, CLIPA, and NUDA together, suggesting that they were dispensable for fundamental functions of MTs. This study provided a comprehensive delineation of the relationship among different +TIPs and their contributions to MT dynamics and unidirectional hyphal expansion in filamentous fungi.

HookA is a novel dynein-early endosome linker critical for cargo movement in vivo

HookA is a novel linker protein that binds to endosomes and to dynein–dynactin and promotes dynein–early endosome interaction in Aspergillus.

Cytoplasmic dynein transports membranous cargoes along microtubules, but the mechanism of dynein–cargo interaction is unclear. From a genetic screen, we identified a homologue of human Hook proteins, HookA, as a factor required for dynein-mediated early endosome movement in the filamentous fungus Aspergillus nidulans. HookA contains a putative N-terminal microtubule-binding domain followed by coiled-coil domains and a C-terminal cargo-binding domain, an organization reminiscent of cytoplasmic linker proteins. HookA–early endosome interaction occurs independently of dynein–early endosome interaction and requires the C-terminal domain. Importantly, HookA interacts with dynein and dynactin independently of HookA–early endosome interaction but dependent on the N-terminal part of HookA. Both dynein and the p25 subunit of dynactin are required for the interaction between HookA and dynein–dynactin, and loss of HookA significantly weakens dynein–early endosome interaction, causing a virtually complete absence of early endosome movement. Thus, HookA is a novel linker important for dynein–early endosome interaction in vivo.

Identification of a novel site in the tail of dynein heavy chain important for dynein function in vivo

The minus end-directed microtubule motor cytoplasmic dynein is responsible for the intracellular movements of many organelles, including nuclei and endosomes. The dynein heavy chain contains a C-terminal motor domain and an N-terminal tail domain. The tail binds other dynein subunits and the cargo-interacting dynactin complex but is dispensable for movement of single dynein molecules in vitro. Here, we identified a mutation in the Aspergillus nidulans heavy chain tail domain, nudA(F208V), which causes obvious defects in dynein-mediated nuclear positioning and early endosome movement. Astonishingly, the nudA(F208I) mutation in the same position does not cause the same defects, suggesting that a subtle difference in the size of the amino acid side chain at this position has a significant consequence. Importantly, our biochemical analyses indicate that the nudA(F208V) mutation does not affect dynein subunit interactions and the mutant dynein is also able to bind dynactin and another dynein regulator, NUDF/LIS1. The mutant dynein is able to physically interact with the early endosome cargo, but dynein-mediated early endosome movement away from the hyphal tip occurs at a significantly reduced frequency. Within the small group of early endosomes that move away from the hyphal tip in the mutant, the average speed of movement is lower than that in the wild type. Given the dispensability of the dynein tail in dynein motility in vitro, our results support the notion that the structural integrity of the dynein tail is critical in vivo for the coordination of dynein force production and movement when the motor is heavily loaded.

Analyses of dynein heavy chain mutations reveal complex interactions between dynein motor domains and cellular dynein functions

Cytoplasmic dynein transports cargoes for a variety of crucial cellular functions. However, since dynein is essential in most eukaryotic organisms, the in-depth study of the cellular function of dynein via genetic analysis of dynein mutations has not been practical. Here, we identify and characterize 34 different dynein heavy chain mutations using a genetic screen of the ascomycete fungus Neurospora crassa, in which dynein is nonessential. Interestingly, our studies show that these mutations segregate into five different classes based on the in vivo localization of the mutated dynein motors. Furthermore, we have determined that the different classes of dynein mutations alter vesicle trafficking, microtubule organization, and nuclear distribution in distinct ways and require dynactin to different extents. In addition, biochemical analyses of dynein from one mutant strain show a strong correlation between its in vitro biochemical properties and the aberrant intracellular function of that altered dynein. When the mutations were mapped to the published dynein crystal structure, we found that the three-dimensional structural locations of the heavy chain mutations were linked to particular classes of altered dynein functions observed in cells. Together, our data indicate that the five classes of dynein mutations represent the entrapment of dynein at five separate points in the dynein mechanochemical and transport cycles. We have developed N. crassa as a model system where we can dissect the complexities of dynein structure, function, and interaction with other proteins with genetic, biochemical, and cell biological studies.

Development of monoclonal antibodies to recombinant terrelysin and characterization of expression in Aspergillus terreus

Aspergillus terreus is an emerging pathogen that mostly affects immunocompromised patients, causing infections that are often difficult to manage therapeutically. Current diagnostic strategies are limited to the detection of fungal growth using radiological methods or biopsy, which often does not enable species-specific identification. There is thus a critical need for diagnostic techniques to enable early and specific identification of the causative agent. In this study, we describe monoclonal antibodies (mAbs) developed to a previously described recombinant form of the haemolysin terrelysin. Sixteen hybridomas of various IgG isotypes were generated to the recombinant protein, of which seven demonstrated reactivity to the native protein in hyphal extracts. Cross-reactivity analysis using hyphal extracts from 29 fungal species, including 12 Aspergillus species and five strains of A. terreus, showed that three mAbs (13G10, 15B5 and 10G4) were A. terreus-specific. Epitope analysis demonstrated mAbs 13G10 and 10G4 recognize the same epitope, PSNEFE, while mAb 15B5 recognizes the epitope LYEGQFHS. Time-course studies showed that terrelysin expression was highest during early hyphal growth and dramatically decreased after mycelial expansion. Immunolocalization studies demonstrated that terrelysin was not only localized within the cytoplasm of hyphae but appeared to be more abundant at the hyphal tip. These findings were confirmed in cultures grown at room temperature as well as at 37 °C. Additionally, terrelysin was detected in the supernatant of A. terreus cultures. These observations suggest that terrelysin may be a candidate biomarker for A. terreus infection.

The p25 subunit of the dynactin complex is required for dynein-early endosome interaction

The p25 subunit of the dynactin complex is required for the interaction between cytoplasmic dynein and early endosomes but is not required for dynein-mediated nuclear distribution.

Cytoplasmic dynein transports various cellular cargoes including early endosomes, but how dynein is linked to early endosomes is unclear. We find that the Aspergillus nidulans orthologue of the p25 subunit of dynactin is critical for dynein-mediated early endosome movement but not for dynein-mediated nuclear distribution. In the absence of NUDF/LIS1, p25 deletion abolished the localization of dynein–dynactin to the hyphal tip where early endosomes abnormally accumulate but did not prevent dynein–dynactin localization to microtubule plus ends. Within the dynactin complex, p25 locates at the pointed end of the Arp1 filament with Arp11 and p62, and our data suggest that Arp11 but not p62 is important for p25–dynactin association. Loss of either Arp1 or p25 significantly weakened the physical interaction between dynein and early endosomes, although loss of p25 did not apparently affect the integrity of the Arp1 filament. These results indicate that p25, in conjunction with the rest of the dynactin complex, is important for dynein–early endosome interaction.

Monoclonal antibodies to hyphal exoantigens derived from the opportunistic pathogen Aspergillus terreus

Aspergillus terreus has been difficult to identify in cases of aspergillosis, and clinical identification has been restricted to the broad identification of aspergillosis lesions in affected organs or the detection of fungal carbohydrates. As a result, there is a clinical need to identify species-specific biomarkers that can be used to detect invasive A. terreus disease. Monoclonal antibodies (MAbs) were developed to a partially purified preparation of cytolytic hyphal exoantigens (HEA) derived from A. terreus culture supernatant (CSN). Twenty-three IgG1 isotype murine MAbs were developed and tested for cross-reactivity against hyphal extracts of 54 fungal species. Sixteen MAbs were shown to be specific for A. terreus. HEA were detected in conidia, hyphae, and in CSN of A. terreus. HEA were expressed in high levels in the hyphae during early stages of A. terreus growth at 37°C, whereas at room temperature the expression of HEA peaked by days 4 to 5. Expression kinetics of HEA in CSN showed a lag, with peak levels at later time points at room temperature and 37°C than in hyphal extracts. Serum spiking experiments demonstrated that human serum components do not inhibit detection of the HEA epitopes by MAb enzyme-linked immunosorbent assay (ELISA). Immunoprecipitation and proteomic analysis demonstrated that MAbs 13E11 and 12C4 immunoprecipitated a putative uncharacterized leucine aminopeptidase (Q0CAZ7), while MAb 19B2 recognized a putative dipeptidyl-peptidase V (DPP5). Studies using confocal laser scanning microscopy showed that the uncharacterized leucine aminopeptidase mostly localized to extracellular matrix structures while dipeptidyl-peptidase V was mostly confined to the cytoplasm.

Characterization of Aspergillus nidulans DidB Did2, a non-essential component of the multivesicular body pathway

ESCRT-III heteropolymers mediate membrane protein cargo sorting into multivesicular endosomes for subsequent vacuolar degradation. We studied the localization of largely uncharacterized Aspergillus nidulans ESCRT-III using its key structural component Vps32 and the ‘associated’ component DidB^Did2. Vps32-GFP localizes to motile early endosomes as reported, but predominates in aggregates often associated with vacuoles due to inability to dissociate from endosomes. DidB^Did2 regulating Vps4 (the ATPase disassembling ESCRT-III) is not essential. Consistent with this accessory role, didBΔ is unable to block the MVB sorting of the glutamate transporter AgtA, but increases its steady-state level and mislocalizes a fraction of the permease to the plasma membrane under conditions promoting its vacuolar targeting. didBΔ exacerbates the dominant-negative growth defect resulting from Vps32-GFP over-expression. A proportion of DidB-GFP is detectable in early endosomes colocalizing with RabA^Rab5 and accumulating in nudA1 tips, suggesting that ESCRT-III assembles on endosomes from the early steps of the endocytic pathway.

Regulators of the cytoplasmic dynein motor

Eukaryotic cells use cytoskeletal motor proteins to transport many different intracellular cargos. Numerous kinesins and myosins have evolved to cope with the various transport needs that have arisen during eukaryotic evolution. Surprisingly, a single cytoplasmic dynein (a minus end-directed microtubule motor) carries out similarly diverse transport activities as the many different types of kinesin. How is dynein coupled to its wide range of cargos and how is it spatially and temporally regulated? The answer could lie in the several multifunctional adaptors, including dynactin, lissencephaly 1, nuclear distribution protein E (NUDE) and NUDE-like, Bicaudal D, Rod-ZW10-Zwilch and Spindly, that regulate dynein function and localization.

Dynein light intermediate chain in Aspergillus nidulans is essential for the interaction between heavy and intermediate chains

Cytoplasmic dynein is a complex containing heavy chains (HCs), intermediate chains (ICs), light intermediate chains (LICs), and light chains (LCs). The HCs are responsible for motor activity. The ICs at the tail region of the motor interact with dynactin, which is essential for dynein function. However, functions of other subunits and how they contribute to the assembly of the core complex are not clearly defined. Here, we analyzed in the filamentous fungus Aspergillus nidulans functions of the only LIC and two LCs, RobA (Roadblock/LC7) and TctexA (Tctex1) in dynein-mediated nuclear distribution (nud). Whereas the deletion mutant of tctexA did not exhibit an apparent nud mutant phenotype, the deletion mutant of robA exhibited a nud phenotype at an elevated temperature, which is similar to the previously characterized nudG (LC8) deletion mutant. Remarkably, in contrast to the single mutants, the robA and nudG double deletion mutant exhibits a severe nud phenotype at various temperatures. Thus, functions of these two LC classes overlap to some extent, but the presence of both becomes important under specific conditions. The single LIC, however, is essential for dynein function in nuclear distribution. This is evidenced by the identification of the nudN gene as the LIC coding gene, and by the nud phenotype exhibited by the LIC down-regulating mutant, alcA-LIC. Without a functional LIC, the HC-IC association is significantly weakened, and the HCs could no longer accumulate at the microtubule plus end. Thus, the LIC is essential for the assembly of the core complex of dynein in Aspergillus.

The Aspergillus nidulans kinesin-3 UncA motor moves vesicles along a subpopulation of microtubules

The extremely polarized growth form of filamentous fungi imposes a huge challenge on the cellular transport machinery, because proteins and lipids required for hyphal extension need to be continuously transported to the growing tip. Recently, it was shown that endocytosis is also important for hyphal growth. Here, we found that the Aspergillus nidulans kinesin-3 motor protein UncA transports vesicles and is required for fast hyphal extension. Most surprisingly, UncA-dependent vesicle movement occurred along a subpopulation of microtubules. Green fluorescent protein (GFP)-labeled UncA(rigor) decorated a single microtubule, which remained intact during mitosis, whereas other cytoplasmic microtubules were depolymerized. Mitotic spindles were not labeled with GFP-UncA(rigor) but reacted with a specific antibody against tyrosinated alpha-tubulin. Hence, UncA binds preferentially to detyrosinated microtubules. In contrast, kinesin-1 (conventional kinesin) and kinesin-7 (KipA) did not show a preference for certain microtubules. This is the first example for different microtubule subpopulations in filamentous fungi and the first example for the preference of a kinesin-3 motor for detyrosinated microtubules.

Dynein light intermediate chain: an essential subunit that contributes to spindle checkpoint inactivation

The dynein light intermediate chain (LIC) is a subunit unique to the cytoplasmic form of dynein, but how it contributes to dynein function is not fully understood. Previous work has established that the LIC homodimer binds directly to the dynein heavy chain and may mediate the attachment of dynein to centrosomes and other cargoes. Here, we report our characterization of the LIC in Drosophila. Unlike vertebrates, in which two Lic genes encode multiple subunit isoforms, the Drosophila LIC is encoded by a single gene. We determined that the single LIC polypeptide is phosphorylated, and that different phosphoisoforms can assemble into the dynein motor complex. Our mutational analyses demonstrate that, similar to other dynein subunits, the Drosophila LIC is required for zygotic development, germline specification of the oocyte, and mitotic cell division. We show that RNA interference depletion of LIC in Drosophila S2 cells does not block the recruitment of a dynein complex to kinetochores, but it does delay inactivation of Mad2 signaling and mitotic progression. Our observations suggest the LIC contributes to a broad range of dynein functions.

Arp11 affects dynein-dynactin interaction and is essential for dynein function in Aspergillus nidulans

The dynactin complex contains proteins including p150 that interacts with cytoplasmic dynein and an actin-related protein Arp1 that forms a minifilament. Proteins including Arp11 and p62 locate at the pointed end of the Arp1 filament, but their biochemical functions are unclear (Schroer TA. Dynactin. Annu Rev Cell Dev Biol 2004;20:759-779). In Aspergillus nidulans, loss of Arp11 or p62 causes the same nuclear distribution (nud) defect displayed by dynein mutants, indicating that these pointed-end proteins are essential for dynein function. We constructed a strain with S-tagged p150 of dynactin that allows us to pull down components of the dynactin and dynein complexes. Surprisingly, while the ratio of pulled-down Arp1 to S-p150 in Arp11-depleted cells is clearly lower than that in wild-type cells, the ratio of pulled-down dynein to S-p150 is significantly higher. We further show that the enhanced dynein-dynactin interaction in Arp11-depleted cells is also present in the soluble fraction and therefore is not dependent upon the affinity of these proteins to the membrane. We suggest that loss of the pointed-end proteins alters the Arp1 filament in a way that affects the conformation of p150 required for its proper interaction with the dynein motor.

Dynein-dependent nuclear dynamics affect morphogenesis in Candida albicans by means of the Bub2p spindle checkpoint

Candida albicans, the most prevalent fungal pathogen of humans, grows with multiple morphologies. The dynamics of nuclear movement are similar in wild-type yeast and pseudohyphae: nuclei divide across the bud neck. By contrast, in hyphae, nuclei migrate 10-20 microm into the growing germ tube before dividing. We analyzed the role of the dynein-dynactin complex in hyphal and yeast cells using time-lapse fluorescence microscopy. Cells lacking the heavy chain of cytoplasmic dynein or the p150(Glued) subunit of dynactin were defective in the position and orientation of the spindle. Hyphal cells often failed to deliver a nucleus to the daughter cell, resulting in defects in morphogenesis. Under yeast growth conditions, cultures included a mixture of yeast and pseudohyphal-like cells that exhibited distinctive defects in nuclear dynamics: in yeast, nuclei divided within the mother cell, and the spindle position checkpoint protein Bub2p ensured the delivery of the daughter nucleus to the daughter cell before cytokinesis; in pseudohyphal-like cells, pre-mitotic nuclei migrated into the daughter and no checkpoint ensured return of a nucleus to the mother cell before cytokinesis. Analysis of double mutants indicated that Bub2p also mediated the pre-anaphase arrest and polarization of pseudohyphal-like cells. Thus, Bub2p has two distinct roles in C. albicans cells lacking dynein: it mediates pre-anaphase arrest and it coordinates spindle disassembly with mitotic exit.

Point mutations in the stem region and the fourth AAA domain of cytoplasmic dynein heavy chain partially suppress the phenotype of NUDF/LIS1 loss in Aspergillus nidulans

Cytoplasmic dynein performs multiple cellular tasks but its regulation remains unclear. The dynein heavy chain has a N-terminal stem that binds to other subunits and a C-terminal motor unit that contains six AAA (ATPase associated with cellular activities) domains and a microtubule-binding site located between AAA4 and AAA5. In Aspergillus nidulans, NUDF (a LIS1 homolog) functions in the dynein pathway, and two nudF6 partial suppressors were mapped to the nudA dynein heavy chain locus. Here we identified these two mutations. The nudAL1098F mutation resides in the stem region, and nudAR3086C is in the end of AAA4. These mutations partially suppress the phenotype of nudF deletion but do not suppress the phenotype exhibited by mutants of dynein intermediate chain and Arp1. Surprisingly, the stronger DeltanudF suppressor, nudAR3086C, causes an obvious decrease in the basal level of dynein's ATPase activity and an increase in dynein's distribution along microtubules. Thus, suppression of the DeltanudF phenotype may result from mechanisms other than simply the enhancement of dynein's ATPase activity. The fact that a mutation in the end of AAA4 negatively regulates dynein's ATPase activity but partially compensates for NUDF loss indicates the importance of the AAA4 domain in dynein regulation in vivo.

Depletion of Aspergillus nidulans cotA causes a severe polarity defect which is not suppressed by the nuclear migration mutation nudA2

The Aspergillus nidulans homologue of Neurospora crassa cot-1, cotA, encoding a member of the NDR protein kinase family, has been cloned and expressed under the control of the conditional alcA promoter. Depletion of CotA by repression of the alcA promoter led to a severe growth defect accompanied by loss of polarity. Germlings show greatly enlarged volume of the spores and hyphae, accompanied by an increase in number of nuclei per compartment, though the nucleus/volume ratio is not significantly altered. The depleted CotA phenotype was not suppressed by a nuclear migration mutation nudA2. Double mutants showed an additive, defective phenotype, unlike the suppression of the cot-1 ts mutation by ropy mutations seen in N. crassa, suggesting a different relationship between nuclear migration and the cot signalling pathway in A. nidulans. A functional CotA-GFP fusion protein was found in punctate regions of fluorescence similar to the distribution reported for human NDR2, and as a cap at the hyphal tip.

CLIP-170 homologue and NUDE play overlapping roles in NUDF localization in Aspergillus nidulans

Proteins in the cytoplasmic dynein pathway accumulate at the microtubule plus end, giving the appearance of comets when observed in live cells. The targeting mechanism for NUDF (LIS1/Pac1) of Aspergillus nidulans, a key component of the dynein pathway, has not been clear. Previous studies have demonstrated physical interactions of NUDF/LIS1/Pac1 with both NUDE/NUDEL/Ndl1 and CLIP-170/Bik1. Here, we have identified the A. nidulans CLIP-170 homologue, CLIPA. The clipA deletion did not cause an obvious nuclear distribution phenotype but affected cytoplasmic microtubules in an unexpected manner. Although more microtubules failed to undergo long-range growth toward the hyphal tip at 32 degrees C, those that reached the hyphal tip were less likely to undergo catastrophe. Thus, in addition to acting as a growth-promoting factor, CLIPA also promotes microtubule dynamics. In the absence of CLIPA, green fluorescent protein-labeled cytoplasmic dynein heavy chain, p150(Glued) dynactin, and NUDF were all seen as plus-end comets at 32 degrees C. However, under the same conditions, deletion of both clipA and nudE almost completely abolished NUDF comets, although nudE deletion itself did not cause a dramatic change in NUDF localization. Based on these results, we suggest that CLIPA and NUDE both recruit NUDF to the microtubule plus end. The plus-end localization of CLIPA itself seems to be regulated by different mechanisms under different physiological conditions. Although the KipA kinesin (Kip2/Tea2 homologue) did not affect plus-end localization of CLIPA at 32 degrees C, it was required for enhancing plus-end accumulation of CLIPA at an elevated temperature (42 degrees C).

Role of the spindle-pole-body protein ApsB and the cortex protein ApsA in microtubule organization and nuclear migration in Aspergillus nidulans

Nuclear migration and positioning in Aspergillus nidulans depend on microtubules, the microtubule-dependent motor protein dynein, and auxiliary proteins, two of which are ApsA and ApsB. In apsA and apsB mutants nuclei are clustered and show various kinds of nuclear navigation defects, although nuclear migration itself is still possible. We studied the role of several components involved in nuclear migration through in vivo fluorescence microscopy using fluorescent-protein tagging. Because ApsA localizes to the cell cortex and mitotic spindles were immobile in apsA mutants, we suggest that astral microtubule-cortex interactions are necessary for oscillation and movement of mitotic spindles along hyphae, but not for post-mitotic nuclear migration. Mutation of apsA resulted in longer and curved microtubules and displayed synthetic lethality in combination with the conventional kinesin mutation DeltakinA. By contrast, ApsB localized to spindle-pole bodies (the fungal centrosome), to septa and to spots moving rapidly along microtubules. The number of cytoplasmic microtubules was reduced in apsB mutants in comparison to the wild type, indicating that cytoplasmic microtubule nucleation was affected, whereas mitotic spindle formation appeared normal. Mutation of apsB suppressed dynein null mutants, whereas apsA mutation had no effect. We suggest that nuclear positioning defects in the apsA and apsB mutants are due to different effects on microtbule organisation. A model of spindle-pole body led nuclear migration and the roles of dynein and microtubules are discussed.

Cytoplasmic dynein's mitotic spindle pole localization requires a functional anaphase-promoting complex, gamma-tubulin, and NUDF/LIS1 in Aspergillus nidulans

In Aspergillus nidulans, cytoplasmic dynein and NUDF/LIS1 are found at the spindle poles during mitosis, but they seem to be targeted to this location via different mechanisms. The spindle pole localization of cytoplasmic dynein requires the function of the anaphase-promoting complex (APC), whereas that of NUDF does not. Moreover, although NUDF's localization to the spindle poles does not require a fully functional dynein motor, the function of NUDF is important for cytoplasmic dynein's targeting to the spindle poles. Interestingly, a gamma-tubulin mutation, mipAR63, nearly eliminates the localization of cytoplasmic dynein to the spindle poles, but it has no apparent effect on NUDF's spindle pole localization. Live cell analysis of the mipAR63 mutant revealed a defect in chromosome separation accompanied by unscheduled spindle elongation before the completion of anaphase A, suggesting that gamma-tubulin may recruit regulatory proteins to the spindle poles for mitotic progression. In A. nidulans, dynein is not apparently required for mitotic progression. In the presence of a low amount of benomyl, a microtubule-depolymerizing agent, however, a dynein mutant diploid strain exhibits a more pronounced chromosome loss phenotype than the control, indicating that cytoplasmic dynein plays a role in chromosome segregation.

Cytoplasmic dynein in fungi: insights from nuclear migration

Cytoplasmic dynein is a microtubule motor that mediates various biological processes, including nuclear migration and organelle transport, by moving on microtubules while associated with various cellular structures. The association of dynein with cellular structures and the activation of its motility are crucial steps in dynein-dependent processes. However, the mechanisms involved remain largely unknown. In fungi, dynein is required for nuclear migration. In budding yeast, nuclear migration is driven by the interaction of astral microtubules with the cell cortex; the interaction is mediated by dynein that is probably associated with the cortex. Recent studies suggest that budding yeast dynein is first recruited to microtubules, then delivered to the cortex by microtubules and finally activated by association with the cortex. Nuclear migration in many other fungi is probably driven by a similar mechanism. Recruitment of dynein to microtubules and its subsequent activation upon association with cellular structures are perhaps common to many dynein-dependent eukaryotic processes, including organelle transport.

A role for cytoplasmic dynein and LIS1 in directed cell movement

Cytoplasmic dynein has been implicated in numerous aspects of intracellular movement. We recently found dynein inhibitors to interfere with the reorientation of the microtubule cytoskeleton during healing of wounded NIH3T3 cell monolayers. We now find that dynein and its regulators dynactin and LIS1 localize to the leading cell cortex during this process. In the presence of serum, bright diffuse staining was observed in regions of active ruffling. This pattern was abolished by cytochalasin D, and was not observed in cells treated with lysophosphatidic acid, conditions which allow microtubule reorientation but not forward cell movement. Under the same conditions, using total internal reflection fluorescence microscopy, clear punctate dynein/dynactin containing structures were observed along the sides and at the tips of microtubules at the leading edge. Overexpression of dominant negative dynactin and LIS1 cDNAs or injection of antidynein antibody interfered with the rate of cell migration. Together, these results implicate a leading edge cortical pool of dynein in both early and persistent steps in directed cell movement.

A class-V myosin required for mating, hyphal growth, and pathogenicity in the dimorphic plant pathogen Ustilago maydis

In the early stages of plant infection, yeast-like haploid sporidia of Ustilago maydis respond to pheromone secreted by compatible partners by forming conjugation tubes. These then fuse to generate a dikaryotic hypha that forms appressoria to penetrate the host plant. As a first step toward understanding the structural requirements for these transitions, we have identified myo5, which encodes a class-V myosin. Analysis of conditional and null mutants revealed that Myo5 plays nonessential roles in cytokinesis and morphogenesis in sporidia and is required for hyphal morphology. Consistent with a role in morphogenesis, a functional green fluorescent protein-Myo5 fusion protein localized to the bud tip and the hyphal apex as well as to the septa and the spore wall during later stages of infection. However, the loss of Myo5 did not affect the tip growth of hyphae and sporidia. By contrast, Myo5 was indispensable for conjugation tube formation. Furthermore, myo5 mutants were impaired in the perception of pheromones, which indicates a particular importance of Myo5 in the mating process. Consequently, few mutant hyphae were formed that penetrated the plant epidermis but did not continue invasive growth. These results indicate a crucial role of Myo5 in the morphogenesis, dimorphic switch, and pathogenicity of U. maydis.

The Aspergillus nidulans alcA promoter drives tightly regulated conditional gene expression in Aspergillus fumigatus permitting validation of essential genes in this human pathogen

Aspergillus fumigatus causes invasive aspergillosis, a mycosis that is usually fatal in immunocompromised patients. Functional genomics in this fungus will aid the discovery of novel antifungal drugs to treat invasive aspergillosis. However, there is still a need for appropriate molecular genetic tools to facilitate such functional studies. Here, we describe the use of a conditional gene expression system allowing the identification of novel therapeutic targets through validation of essential genes in A. fumigatus. This system is based on the capacity of the Aspergillus nidulans alcA promoter (alcA(p)) to tightly regulate gene expression in this fungus. Conditionally regulated gene expression in A. fumigatus was demonstrated by transcriptional and phenotypic analyses of strains expressing a nuclear migration gene with a terminal phenotype, the A. fumigatus nudC gene, under control of this promoter. This conditional expression system, the first one described in A. fumigatus, will also be useful for investigating the function of essential genes by altering the threonine/glucose ratio in the growth medium.

Accumulation of cytoplasmic dynein and dynactin at microtubule plus ends in Aspergillus nidulans is kinesin dependent

The mechanism(s) by which microtubule plus-end tracking proteins are targeted is unknown. In the filamentous fungus Aspergillus nidulans, both cytoplasmic dynein and NUDF, the homolog of the LIS1 protein, localize to microtubule plus ends as comet-like structures. Herein, we show that NUDM, the p150 subunit of dynactin, also forms dynamic comet-like structures at microtubule plus ends. By examining proteins tagged with green fluorescent protein in different loss-of-function mutants, we demonstrate that dynactin and cytoplasmic dynein require each other for microtubule plus-end accumulation, and the presence of cytoplasmic dynein is also important for NUDF's plus-end accumulation. Interestingly, deletion of NUDF increases the overall accumulation of dynein and dynactin at plus ends, suggesting that NUDF may facilitate minus-end-directed dynein movement. Finally, we demonstrate that a conventional kinesin, KINA, is required for the microtubule plus-end accumulation of cytoplasmic dynein and dynactin, but not of NUDF.

Roles of NUDE and NUDF proteins of Aspergillus nidulans: insights from intracellular localization and overexpression effects

The NUDF protein of the filamentous fungus Aspergillus nidulans functions in the cytoplasmic dynein pathway. It binds several proteins, including the NUDE protein. Green fluorescent protein-tagged NUDF and NUDA (dynein heavy chain) localize to linearly moving dashes ("comets") that coincide with microtubule ends. Herein, deletion of the nudE gene did not eliminate the comets of NUDF and NUDA, but affected the behavior of NUDA. Comets were also observed with the green fluorescent protein-tagged NUDE and its nonfunctional C-terminal domain. In addition, overexpressed NUDA and NUDE accumulated in specks that were either immobile or bounced randomly. Neither comets nor specks were observed with the functional N-terminal domain of NUDE, indicating that these structures are not essential for NUDE function. Furthermore, NUDF overproduction totally suppressed deletion of the nudE gene. This implies that the function of NUDE is secondary to that of NUDF. Unexpectedly, NUDF overproduction inhibited one conditional nudA mutant and all tested apsA mutants. An allele-specific interaction between the nudF and nudA genes is consistent with a direct interaction between NUDF and dynein heavy chain. Because APSA and its yeast homolog Num1p are cortical proteins, an interaction between the nudF and apsA genes suggests a role for NUDF at the cell cortex.

Cytoplasmic dynein intermediate chain and heavy chain are dependent upon each other for microtubule end localization in Aspergillus nidulans

The multisubunit microtubule motor, cytoplasmic dynein, targets to various subcellular locations in eukaryotic cells for various functions. The cytoplasmic dynein heavy chain (HC) contains the microtubule binding and ATP binding sites for motor function, whereas the intermediate chain (IC) is implicated in the in vivo targeting of the HC. Concerning any targeting event, it is not known whether the IC has to form a complex with the HC for targeting or whether the IC can target to a site independently of the HC. In the filamentous fungus Aspergillus nidulans, the dynein HC is localized to the ends of microtubules near the hyphal tip. In this study, we demonstrate that our newly identified dynein IC in A. nidulans is also localized to microtubule ends and is required for HC's localization to microtubule ends in living cells. With the combination of two reagents, an HC loss-of function mutant and the green fluorescent protein (GFP)-fused IC that retains its function, we show that the IC's localization to microtubule ends also requires HC, suggesting that cytoplasmic dynein HC-IC complex formation is important for microtubule end targeting. In addition, we show that the HC localization is not apparently altered in the deletion mutant of NUDF, a LIS1-like protein that interacts directly with the ATP-binding domain of the HC. Our study suggests that, although HC-IC association is important for the targeting of dynein to microtubule ends, other essential components, such as NUDF, may interact with the targeted dynein complex to produce full motor activities in vivo.

Dynein at the cortex

Cytoplasmic dynein is a minus end directed microtubule motor protein with numerous functions during interphase and mitosis. Recent evidence has identified several roles mediated by a fraction of cytoplasmic dynein associated with the cell cortex. So far, these include nuclear migration, mitotic spindle orientation, and cytoskeletal reorientation during wound healing, but others are likely. The possibility that a cortically bound form of dynein might represent its most ancient evolutionary state is discussed.

Dynamic behavior of microtubules during dynein-dependent nuclear migrations of meiotic prophase in fission yeast

During meiotic prophase in fission yeast, the nucleus migrates back and forth between the two ends of the cell, led by the spindle pole body (SPB). This nuclear oscillation is dependent on astral microtubules radiating from the SPB and a microtubule motor, cytoplasmic dynein. Here we have examined the dynamic behavior of astral microtubules labeled with the green fluorescent protein during meiotic prophase with the use of optical sectioning microscopy. During nuclear migrations, the SPB mostly follows the microtubules that extend toward the cell cortex. SPB migrations start when these microtubules interact with the cortex and stop when they disappear, suggesting that these microtubules drive nuclear migrations. The microtubules that are followed by the SPB often slide along the cortex and are shortened by disassembly at their ends proximal to the cortex. In dynein-mutant cells, where nuclear oscillations are absent, the SPB never migrates by following microtubules, and microtubule assembly/disassembly dynamics is significantly altered. Based on these observations, together with the frequent accumulation of dynein at a cortical site where the directing microtubules interact, we propose a model in which dynein drives nuclear oscillation by mediating cortical microtubule interactions and regulating the dynamics of microtubule disassembly at the cortex.

Genetic evidence for a microtubule-destabilizing effect of conventional kinesin and analysis of its consequences for the control of nuclear distribution in Aspergillus nidulans

Conventional kinesin is a microtubule-dependent motor protein believed to be involved in a variety of intracellular transport processes. In filamentous fungi, conventional kinesin has been implicated in different processes, such as vesicle migration, polarized growth, nuclear distribution, mitochondrial movement and vacuole formation. To gain further insights into the functions of this kinesin motor, we identified and characterized the conventional kinesin gene, kinA, of the established model organism Aspergillus nidulans. Disruption of the gene leads to a reduced growth rate and a nuclear positioning defect, resulting in nuclear cluster formation. These clusters are mobile and display a dynamic behaviour. The mutant phenotypes are pronounced at 37 degrees C, but rescued at 25 degrees C. The hyphal growth rate at 25 degrees C was even higher than that of the wild type at the same temperature. In addition, kinesin-deficient strains were less sensitive to the microtubule destabilizing drug benomyl, and disruption of conventional kinesin suppressed the cold sensitivity of an alpha-tubulin mutation (tubA4). These results suggest that conventional kinesin of A. nidulans plays a role in cytoskeletal dynamics, by destabilizing microtubules. This new role of conventional kinesin in microtubule stability could explain the various phenotypes observed in different fungi.

Cytoplasmic dynein light intermediate chain is required for discrete aspects of mitosis in Caenorhabditis elegans

We describe phenotypic characterization of dli-1, the Caenorhabditis elegans homolog of cytoplasmic dynein light intermediate chain (LIC), a subunit of the cytoplasmic dynein motor complex. Animals homozygous for loss-of-function mutations in dli-1 exhibit stochastic failed divisions in late larval cell lineages, resulting in zygotic sterility. dli-1 is required for dynein function during mitosis. Depletion of the dli-1 gene product through RNA-mediated gene interference (RNAi) reveals an early embryonic requirement. One-cell dli-1(RNAi) embryos exhibit failed cell division attempts, resulting from a variety of mitotic defects. Specifically, pronuclear migration, centrosome separation, and centrosome association with the male pronuclear envelope are defective in dli-1(RNAi) embryos. Meiotic spindle formation, however, is not affected in these embryos. DLI-1, like its vertebrate homologs, contains a putative nucleotide-binding domain similar to those found in the ATP-binding cassette transporter family of ATPases as well as other nucleotide-binding and -hydrolyzing proteins. Amino acid substitutions in a conserved lysine residue, known to be required for nucleotide binding, confers complete rescue in a dli-1 mutant background, indicating this is not an essential domain for DLI-1 function.

A split motor domain in a cytoplasmic dynein

The heavy chain of dynein forms a globular motor domain that tightly couples the ATP-cleavage region and the microtubule-binding site to transform chemical energy into motion along the cytoskeleton. Here we show that, in the fungus Ustilago maydis, two genes, dyn1 and dyn2, encode the dynein heavy chain. The putative ATPase region is provided by dyn1, while dyn2 includes the predicted microtubule-binding site. Both genes are located on different chromosomes, are transcribed into independent mRNAs and are translated into separate polypeptides. Both Dyn1 and Dyn2 co-immunoprecipitated and co-localized within growing cells, and Dyn1-Dyn2 fusion proteins partially rescued mutant phenotypes, suggesting that both polypeptides interact to form a complex. In cell extracts the Dyn1-Dyn2 complex dissociated, and microtubule affinity purification indicated that Dyn1 or associated polypeptides bind microtubules independently of Dyn2. Both Dyn1 and Dyn2 were essential for cell survival, and conditional mutants revealed a common role in nuclear migration, cell morphogenesis and microtubule organization, indicating that the Dyn1-Dyn2 complex serves multiple cellular functions.

The Aspergillus cytoplasmic dynein heavy chain and NUDF localize to microtubule ends and affect microtubule dynamics

Cytoplasmic dynein is a multisubunit, minus end-directed microtubule motor that uses dynactin as an accessory complex to perform various in vivo functions including vesicle transport, spindle assembly, and nuclear distribution [1]. We previously showed that in the filamentous fungus Aspergillus nidulans, a GFP-tagged cytoplasmic dynein heavy chain (NUDA) forms comet-like structures that exhibited microtubule-dependent movement toward and back from the hyphal tip [2]. Here we demonstrate that another protein in the NUDA pathway, NUDF, which is homologous to the human LIS1 protein involved in brain development [3, 4], also exhibits such dynamic behavior. Both NUDA and NUDF are located at the ends of microtubules, and this observation suggests that the observed dynamic behavior is due to their association with the dynamic microtubule ends. To address whether NUDA and NUDF play a role in regulating microtubule dynamics in vivo, we constructed a GFP-labeled alpha-tubulin strain and used it to compare microtubule dynamics in vivo in wild-type A. nidulans versus temperature-sensitive loss-of-function mutants of nudA and nudF. The mutants showed a lower frequency of microtubule catastrophe, a lower rate of shrinkage during catastrophe, and a lower frequency of rescue. The microtubules in the mutant cells also paused longer at the hyphal tip than wild-type microtubules. These results indicate that cytoplasmic dynein and the LIS1 homolog NUDF affect microtubule dynamics in vivo.

Protein-protein interactions, cytoskeletal regulation and neuronal migration

Neuronal migration, like the migration of many cell types, requires an extensive rearrangement of cell shape, mediated by changes in the cytoskeleton. The genetic analysis of human brain malformations has identified several biochemical players in this process, including doublecortin (DCX) and LIS1, mutations of which cause a profound migratory disturbance known as lissencephaly ('smooth brain') in humans. Studies in mice have identified additional molecules such as Cdk5, P35, Reelin, Disabled and members of the LDL superfamily of receptors. Understanding the cell biology of these molecules has been a challenge, and it is not known whether they function in related biochemical pathways or in very distinct processes. The last year has seen rapid advances in the biochemical analysis of several such molecules. This analysis has revealed roles for some of these molecules in cytoskeletal regulation and has shown an unexpected conservation of the genetic pathways that regulate neuronal migration in humans and nuclear movement in simple eukaryotic organisms.

Cytoplasmic dynein is required to oppose the force that moves nuclei towards the hyphal tip in the filamentous ascomycete Ashbya gossypii

We have followed the migration of GFP-labelled nuclei in multinucleate hyphae of Ashbya gossypii. For the first time we could demonstrate that the mode of long range nuclear migration consists of oscillatory movements of nuclei with, on average, higher amplitudes in the direction of the growing tip. We could also show that mitotic division proceeds at a constant rate of 0. 64 microm/minute which differs from the biphasic kinetics described for the yeast Saccharomyces cerevisiae. Furthermore we were able to identify the microtubule-based motor dynein as a key element in the control of long range nuclear migration. For other filamentous fungi it had already been demonstrated that inactivating mutations in dynein led to severe problems in nuclear migration, i.e. generation of long nuclei-free hyphal tips and clusters of nuclei throughout the hyphae. This phenotype supported the view that dynein is important for the movement of nuclei towards the tip. In A. gossypii the opposite seems to be the case. A complete deletion of the dynein heavy chain gene leads to nuclear clusters exclusively at the hyphal tips and to an essentially nucleus-free network of hyphal tubes and branches. Anucleate hyphae and branches in the vicinity of nuclear clusters show actin cables and polarized actin patches, as well as microtubules. The slow growth of this dynein null mutant could be completely reverted to wild-type-like growth in the presence of benomyl, which can be explained by the observed redistribution of nuclei in the hyphal network.

NudE-L, a novel Lis1-interacting protein, belongs to a family of vertebrate coiled-coil proteins

The LIS1-encoded protein (Lis1) plays a role in brain development because a hemizygous deletion or mutation of the human gene causes neuronal migration disorders, such as Miller-Dieker syndrome (MDS) or isolated lissencephaly sequence (ILS). Using a yeast two-hybrid screen, we have isolated a novel protein that interacts with mouse Lis1 (mLis1) which is termed mouse NudE-like protein (mNudE-L) because of its 49% amino acid conservation with NudE, a protein involved in nuclear migration in Aspergillus nidulans. GST pull-down assays and co-immunoprecipitation of fusion proteins expressed in mammalian cells confirmed the interaction of mLis1 and mNudE-L. mNudE-L gives rise to a approximately 2.3 kb mRNA and encodes an ORF corresponding to approximately 38 kDa protein. The overall amino acid sequence of mNudE-L is 49-95% identical to proteins found in a variety of organisms, thus establishing mNudE-L as a new member of a protein family. The hallmark of this family is an N-terminal region predicted to form a coiled-coil domain. We show that mNudE-L and mLis1 are coexpressed in the postnatal and adult cerebral cortices and in the Purkinje neurons of the cerebellum. In contrast to mLis1, mNudE-L transcripts are absent in the mitral cell layer of the olfactory bulb and in the inward migrating granular neurons of the developing cerebellum. Mutant mLis1 proteins modelling mutations found in human lissencephaly patients fail to interact with mNudE-L, raising the possibility that phenotypic changes result, in part, from the inability of mutant Lis1 proteins to interact with the human NudE-L polypeptide.

The Drosophila Lissencephaly1 (DLis1) gene is required for nuclear migration

Nuclear movement is critical for several developmental processes in eukaryotes. Drosophila oogenesis provides a paradigmatic example in which localization of the nucleus generates a source of cellular asymmetry that is used in patterning both the anterior-posterior and the dorsal-ventral axes of the oocyte. In this study we show that mutations in the Drosophila Lissencephaly1 (DLis1) gene result in partial ventralization of the eggshell. DLis1 mutations affect the localization of gurken mRNA and protein in the oocyte. These defects are correlated with incorrect positioning of the oocyte nucleus, suggesting that DLis1 is required for nuclear migration. DLis1 shows significant sequence conservation across the evolutionary spectrum. Fungal cognates of DLis1 are involved in nuclear migration while homologs in humans and mice are implicated in neuronal migration. DLis1 shows genetic interactions with the Glued and Dynein heavy chain subunits of the dynein/dynactin complex, supporting the idea that the Lis1 family of proteins plays a role in microtubule motor-based nuclear motility.

Direct association of LIS1, the lissencephaly gene product, with a mammalian homologue of a fungal nuclear distribution protein, rNUDE

LIS1 is a product of the causative gene for type I lissencephaly characterized by a smooth brain surface due to a defect in neuronal migration during brain development and a regulatory subunit of platelet-activating factor acetylhydrolase (PAF-AH). It is also a mammalian homologue of the fungal nuclear distribution (nud) gene, nudF, which controls the migration of fungal nuclei. Using the two-hybrid system, we identified a novel LIS1-interacting protein, rat NUDE (rNUDE), and found that it is a mammalian homologue of another fungal nud gene product, NUDE, and Xenopus mitotic phosphoprotein 43 which is phosphorylated in a cell cycle-dependent manner. rNUDE and the catalytic subunits of PAF-AH interact with the N- and C-termini of LIS1, respectively. However, these proteins, instead of simultaneously binding to LIS1, appeared to bind to LIS1 in a competitive manner. These results suggest that LIS1 functions in nuclear migration by interacting with multiple intracellular proteins in mammals.

The LIS1-related NUDF protein of Aspergillus nidulans interacts with the coiled-coil domain of the NUDE/RO11 protein

The nudF gene of the filamentous fungus Aspergillus nidulans acts in the cytoplasmic dynein/dynactin pathway and is required for distribution of nuclei. NUDF protein, the product of the nudF gene, displays 42% sequence identity with the human protein LIS1 required for neuronal migration. Haploinsufficiency of the LIS1 gene causes a malformation of the human brain known as lissencephaly. We screened for multicopy suppressors of a mutation in the nudF gene. The product of the nudE gene isolated in the screen, NUDE, is a homologue of the nuclear distribution protein RO11 of Neurospora crassa. The highly conserved NH(2)-terminal coiled-coil domain of the NUDE protein suffices for protein function when overexpressed. A similar coiled-coil domain is present in several putative human proteins and in the mitotic phosphoprotein 43 (MP43) of X. laevis. NUDF protein interacts with the Aspergillus NUDE coiled-coil in a yeast two-hybrid system, while human LIS1 interacts with the human homologue of the NUDE/RO11 coiled-coil and also the Xenopus MP43 coiled-coil. In addition, NUDF coprecipitates with an epitope-tagged NUDE. The fact that NUDF and LIS1 interact with the same protein domain strengthens the notion that these two proteins are functionally related.