Cytoplasmic dynamics of the general nuclear import machinery in apically growing syncytial cells

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.

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, 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 an Arp1 (actin-related protein 1) mini-filament associated with a pointed-end sub-complex. VezA physically interacts with dynactin either directly or indirectly via the Arp1 mini-filament and its pointed-end sub-complex. Loss of VezA causes a defect in dynactin integrity, most likely by affecting the connection between the Arp1 mini-filament and its pointed-end sub-complex. Using various dynactin mutants, we further revealed that assembly of the dynactin complex must be highly coordinated. Together, these results shed important new light on dynactin assembly in vivo.

The splicing-factor Prp40 affects dynein-dynactin function in Aspergillus nidulans

The multi-component cytoplasmic dynein transports cellular cargoes with the help of another multi-component complex dynactin, but we do not know enough about factors that may affect the assembly and functions of these proteins. From a genetic screen for mutations affecting early-endosome distribution in Aspergillus nidulans, we identified the prp40A^L438* mutation in Prp40A, a homologue of Prp40, an essential RNA-splicing factor in the budding yeast. Prp40A is not essential for splicing, although it associates with the nuclear splicing machinery. The prp40A^L438* mutant is much healthier than the ∆prp40A mutant, but both mutants exhibit similar defects in dynein-mediated early-endosome transport and nuclear distribution. In the prp40A^L438* mutant, the frequency but not the speed of dynein-mediated early-endosome transport is decreased, which correlates with a decrease in the microtubule plus-end accumulations of dynein and dynactin. Within the dynactin complex, the actin-related protein Arp1 forms a mini-filament. In a pull-down assay, the amount of Arp1 pulled down with its pointed-end protein Arp11 is lowered in the prp40A^L438* mutant. In addition, we found from published interactome data that a mammalian Prp40 homologue PRPF40A interacts with Arp1. Thus, Prp40 homologues may regulate the assembly or function of dynein–dynactin and their mechanisms deserve to be further studied.

Developmental regulators FlbE/D orchestrate the polarity site-to-nucleus dynamics of the fungal bZIP transcription factor FlbB

Permanently polarized cells have developed transduction mechanisms linking polarity sites with gene regulation in the nucleus. In neurons, one mechanism is based on long-distance retrograde migration of transcription factors (TFs). Aspergillus nidulans FlbB is the only known fungal TF shown to migrate retrogradely to nuclei from the polarized region of fungal cells known as hyphae. There, FlbB controls developmental transitions by triggering the production of asexual multicellular structures. FlbB dynamics in hyphae is orchestrated by regulators FlbE and FlbD. At least three FlbE domains are involved in the acropetal transport of FlbB, with a final MyoE/actin filament-dependent step from the subapex to the apex. Experiments employing a T2A viral peptide-containing chimera (FlbE::mRFP::T2A::FlbB::GFP) suggest that apical FlbB/FlbE interaction is inhibited to initiate a dynein-dependent FlbB transport to nuclei. FlbD controls the nuclear accumulation of FlbB through a cMyb domain and a C-terminal LxxLL motif. Overall, results elucidate a highly dynamic pattern of FlbB interactions, which enable timely developmental induction. Furthermore, this system establishes a reference for TF-based long-distance signaling in permanently polarized cells.

Regulation of Aspergillus nidulans CreA-Mediated Catabolite Repression by the F-Box Proteins Fbx23 and Fbx47

The attachment of one or more ubiquitin molecules by SCF (Skp-Cullin-F-box) complexes to protein substrates targets them for subsequent degradation by the 26S proteasome, allowing the control of numerous cellular processes. Glucose-mediated signaling and subsequent carbon catabolite repression (CCR) are processes relying on the functional regulation of target proteins, ultimately controlling the utilization of this carbon source. In the filamentous fungus Aspergillus nidulans, CCR is mediated by the transcription factor CreA, which modulates the expression of genes encoding biotechnologically relevant enzymes. Although CreA-mediated repression of target genes has been extensively studied, less is known about the regulatory pathways governing CCR and this work aimed at further unravelling these events. The Fbx23 F-box protein was identified as being involved in CCR and the Δfbx23 mutant presented impaired xylanase production under repressing (glucose) and derepressing (xylan) conditions. Mass spectrometry showed that Fbx23 is part of an SCF ubiquitin ligase complex that is bridged via the GskA protein kinase to the CreA-SsnF-RcoA repressor complex, resulting in the degradation of the latter under derepressing conditions. Upon the addition of glucose, CreA dissociates from the ubiquitin ligase complex and is transported into the nucleus. Furthermore, casein kinase is important for CreA function during glucose signaling, although the exact role of phosphorylation in CCR remains to be determined. In summary, this study unraveled novel mechanistic details underlying CreA-mediated CCR and provided a solid basis for studying additional factors involved in carbon source utilization which could prove useful for biotechnological applications.IMPORTANCE The production of biofuels from plant biomass has gained interest in recent years as an environmentally friendly alternative to production from petroleum-based energy sources. Filamentous fungi, which naturally thrive on decaying plant matter, are of particular interest for this process due to their ability to secrete enzymes required for the deconstruction of lignocellulosic material. A major drawback in fungal hydrolytic enzyme production is the repression of the corresponding genes in the presence of glucose, a process known as carbon catabolite repression (CCR). This report provides previously unknown mechanistic insights into CCR through elucidating part of the protein-protein interaction regulatory system that governs the CreA transcriptional regulator in the reference organism Aspergillus nidulans in the presence of glucose and the biotechnologically relevant plant polysaccharide xylan.

Nuclear movement in fungi

Nuclear movement within a cell occurs in a variety of eukaryotic organisms including yeasts and filamentous fungi. Fungal molecular genetic studies identified the minus-end-directed microtubule motor cytoplasmic dynein as a critical protein for nuclear movement or orientation of the mitotic spindle contained in the nucleus. Studies in the budding yeast first indicated that dynein anchored at the cortex via its anchoring protein Num1 exerts pulling force on an astral microtubule to orient the anaphase spindle across the mother-daughter axis before nuclear division. Prior to anaphase, myosin V interacts with the plus end of an astral microtubule via Kar9-Bim1/EB1 and pulls the plus end along the actin cables to move the nucleus/spindle close to the bud neck. In addition, pushing or pulling forces generated from cortex-linked polymerization or depolymerization of microtubules drive nuclear movements in yeasts and possibly also in filamentous fungi. In filamentous fungi, multiple nuclei within a hyphal segment undergo dynein-dependent back-and-forth movements and their positioning is also influenced by cytoplasmic streaming toward the hyphal tip. In addition, nuclear movement occurs at various stages of fungal development and fungal infection of plant tissues. This review discusses our current understanding on the mechanisms of nuclear movement in fungal organisms, the importance of nuclear positioning and the regulatory strategies that ensure the proper positioning of nucleus/spindle.

Neurons show the path: tip-to-nucleus communication in filamentous fungal development and pathogenesis

Multiple fungal species penetrate substrates and accomplish host invasion through the fast, permanent and unidirectional extension of filamentous cells known as hyphae. Polar growth of hyphae results, however, in a significant increase in the distance between the polarity site, which also receives the earliest information about ambient conditions, and nuclei, where adaptive responses are executed. Recent studies demonstrate that these long distances are overcome by signal transduction pathways which convey sensory information from the polarity site to nuclei, controlling development and pathogenesis. The present review compares the striking connections of the mechanisms for long-distance communication in hyphae with those from neurons, and discusses the importance of their study in order to understand invasion and dissemination processes of filamentous fungi, and design strategies for developmental control in the future.

Tip-to-nucleus migration dynamics of the asexual development regulator FlbB in vegetative cells

In Aspergillus nidulans, asexual differentiation requires the presence of the transcription factor FlbB at the cell tip and apical nuclei. Understanding the relationship between these two pools is crucial for elucidating the biochemical processes mediating conidia production. Tip-to-nucleus communication was demonstrated by photo-convertible FlbB::Dendra2 visualization. Tip localization of FlbB depends on Cys382 in the C-terminus and the bZIP DNA-binding domain in the N-terminus. FlbE, a critical FlbB interactor, binds the bZIP domain. Furthermore, the absence of FlbE results in loss of tip localization but not nuclear accumulation. flbE deletion also abrogates transcriptional activity indicating that FlbB gains transcriptional competence from interactions with FlbE at the tip. Finally, a bipartite nuclear localization signal is required for nuclear localization of FlbB. Those motifs of FlbB may play various roles in the sequence of events necessary for the distribution and activation of this transcriptionally active developmental factor. The tip accumulation, FlbE-dependent activation, transport and nuclear import sketch out a process of relaying an environmentally triggered signal from the tip to the nuclei. As the first known instance of transcription factor-mediated tip-to-nucleus communication in filamentous fungi, this provides a general framework for analyses focused on elucidating the set of molecular mechanisms coupling apical signals to transcriptional events.