Conditional inactivation ofAspergillus nidulans sarASAR1uncovers the morphogenetic potential of regulating endoplasmic reticulum (ER) exit

Visualizing organelles with recombinant fluorescent proteins in the white-rot fungus Pleurotus ostreatus

White-rot fungi secrete numerous enzymes involved in lignocellulose degradation. However, the secretory mechanisms or pathways, including protein synthesis, folding, modification, and traffic, have not been well studied. In the first place, few experimental tools for molecular cell biological studies have been developed. As the first step toward investigating the mechanisms underlying protein secretion, this study visualized organelles and transport vesicles involved in secretory mechanisms with fluorescent proteins in living cells of the white-rot fungus Pleurotus ostreatus (agaricomycete). To this end, each plasmid containing the expression cassette for fluorescent protein [enhanced green fluorescent protein (EGFP) or mCherry] fused with each protein that may be localized in the endoplasmic reticulum (ER), Golgi, or secretory vesicles (SVs) was introduced into P. ostreatus strain PC9. Fluorescent microscopic analyses of the obtained hygromycin-resistant transformants suggested that Sec13-EGFP and Sec24-EGFP visualize the ER; Sec24-EGFP, mCherry-Sed5, and mCherry-Rer1 visualize the compartment likely corresponding to early Golgi and/or the ER-Golgi intermediate compartment; EGFP/mCherry-pleckstrin homology (PH) visualizes possible late Golgi; and EGFP-Seg1 and mCherry-Rab11 visualize SVs. This study successfully visualized mitochondria and nuclei, thus providing useful tools for future molecular cell biological studies on lignocellulose degradation by P. ostreatus. Furthermore, some differences in the Golgi compartment or apparatus and the ER-Golgi intermediate of P. ostreatus compared to other fungi were also suggested.

The Uso1 globular head interacts with SNAREs to maintain viability even in the absence of the coiled-coil domain

Uso1/p115 and RAB1 tether ER-derived vesicles to the Golgi. Uso1/p115 contains a globular-head-domain (GHD), a coiled-coil (CC) mediating dimerization/tethering, and a C-terminal region (CTR) interacting with golgins. Uso1/p115 is recruited to vesicles by RAB1. Genetic studies placed Uso1 paradoxically acting upstream of, or in conjunction with RAB1 (Sapperstein et al., 1996). We selected two missense mutations in uso1 resulting in E6K and G540S in the GHD that rescued lethality of rab1-deficient Aspergillus nidulans. The mutations are phenotypically additive, their combination suppressing the complete absence of RAB1, which emphasizes the key physiological role of the GHD. In living hyphae Uso1 recurs on puncta (60 s half-life) colocalizing partially with the Golgi markers RAB1, Sed5, and GeaA/Gea1/Gea2, and totally with the retrograde cargo receptor Rer1, consistent with Uso1 dwelling in a very early Golgi compartment from which ER residents reaching the Golgi recycle back to the ER. Localization of Uso1, but not of Uso1E6K/G540S, to puncta is abolished by compromising RAB1 function, indicating that E6K/G540S creates interactions bypassing RAB1. That Uso1 delocalization correlates with a decrease in the number of Gea1 cisternae supports that Uso1-and-Rer1-containing puncta are where the protein exerts its physiological role. In S-tag-coprecipitation experiments, Uso1 is an associate of the Sed5/Bos1/Bet1/Sec22 SNARE complex zippering vesicles with the Golgi, with Uso1E6K/G540S showing a stronger association. Using purified proteins, we show that Bos1 and Bet1 bind the Uso1 GHD directly. However, Bet1 is a strong E6K/G540S-independent binder, whereas Bos1 is weaker but becomes as strong as Bet1 when the GHD carries E6K/G540S. G540S alone markedly increases GHD binding to Bos1, whereas E6K causes a weaker effect, correlating with their phenotypic contributions. AlphaFold2 predicts that G540S increases the binding of the GHD to the Bos1 Habc domain. In contrast, E6K lies in an N-terminal, potentially alpha-helical, region that sensitive genetic tests indicate as required for full Uso1 function. Remarkably, this region is at the end of the GHD basket opposite to the end predicted to interact with Bos1. We show that, unlike dimeric full-length and CTR∆ Uso1 proteins, the GHD lacking the CC/CTR dimerization domain, whether originating from bacteria or Aspergillus extracts and irrespective of whether it carries or not E6K/G540S, would appear to be monomeric. With the finding that overexpression of E6K/G540S and wild-type GHD complement uso1∆, our data indicate that the GHD monomer is capable of providing, at least partially, the essential Uso1 functions, and that long-range tethering activity is dispensable. Rather, these findings strongly suggest that the essential role of Uso1 involves the regulation of SNAREs.

Cell wall dynamics stabilize tip growth in a filamentous fungus

Hyphal tip growth allows filamentous fungi to colonize space, reproduce, or infect. It features remarkable morphogenetic plasticity including unusually fast elongation rates, tip turning, branching, or bulging. These shape changes are all driven from the expansion of a protective cell wall (CW) secreted from apical pools of exocytic vesicles. How CW secretion, remodeling, and deformation are modulated in concert to support rapid tip growth and morphogenesis while ensuring surface integrity remains poorly understood. We implemented subresolution imaging to map the dynamics of CW thickness and secretory vesicles in Aspergillus nidulans. We found that tip growth is associated with balanced rates of CW secretion and expansion, which limit temporal fluctuations in CW thickness, elongation speed, and vesicle amount, to less than 10% to 20%. Affecting this balance through modulations of growth or trafficking yield to near-immediate changes in CW thickness, mechanics, and shape. We developed a model with mechanical feedback that accounts for steady states of hyphal growth as well as rapid adaptation of CW mechanics and vesicle recruitment to different perturbations. These data provide unprecedented details on how CW dynamics emerges from material secretion and expansion, to stabilize fungal tip growth as well as promote its morphogenetic plasticity.

Sar1 Interacts with Sec23/Sec24 and Sec13/Sec31 Complexes: Insight into Its Involvement in the Assembly of Coat Protein Complex II in the Microsporidian Nosema bombycis

Microsporidia, as unicellular eukaryotes, also have an endomembrane system for transporting proteins, which is essentially similar to those of other eukaryotes. In eukaryotes, coat protein complex II (COPII) consists of Sar1, Sec23, Sec24, Sec13, and Sec31 and mediates protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus. Sar1 is the central player in the regulation of coat protein complex II vesicle formation in the endoplasmic reticulum. In this study, we successfully cloned the NbSar1, NbSec23-1, NbSec23-2, NbSec24-1, NbSec24-2, NbSec13, NbSec31-1, and NbSec31-2 genes and prepared NbSar1 polyclonal antibody. We found that NbSar1 was localized mainly in the perinuclear cytoplasm of Nosema bombycis by immunofluorescence analysis (IFA). Yeast two-hybrid assays demonstrated that NbSar1 interacts with NbSec23-2, NbSec23-2 interacts with NbSec24-1 or NbSec24-2, NbSec23-1 interacts with NbSec31, and NbSec31 interacts with NbSec13. Moreover, the silencing of NbSar1 by RNA interference resulted in the aberrant expression of NbSar1, NbSec23-1, NbSec24-1, NbSec24-2, NbSec13, NbSec31-1, and NbSec31-2 and significantly inhibited the proliferation of N. bombycis. Altogether, these findings indicated that the subunits of coat protein complex II work together to perform functions in the proliferation of N. bombycis and that NbSar1 may play a crucial role in coat protein complex II vesicle formation. IMPORTANCE As eukaryotes, microsporidia have retained the endomembrane system for transporting and sorting proteins throughout their evolution. Whether the microsporidia form coat protein complex II (COPII) vesicles to transport cargo proteins and whether they play other roles besides cargo transport are not fully explained at present. Our results showed that NbSar1, NbSec23-1/NbSec23-2, NbSec24-1/NbSec24-2, NbSec13, and NbSec31 might be assembled to form COPII in the ER of N. bombycis, and the functions of COPII are also closely related to the proliferation of N. bombycis, this may be a new target for the prevention of pébrine disease of the silkworm.

The type V myosin-containing complex HUM is a RAB11 effector powering movement of secretory vesicles

In the apex-directed RAB11 exocytic pathway of Aspergillus nidulans, kinesin-1/KinA conveys secretory vesicles (SVs) to the hyphal tip, where they are transferred to the type V myosin MyoE. MyoE concentrates SVs at an apical store located underneath the PM resembling the presynaptic active zone. A rod-shaped RAB11 effector, UDS1, and the intrinsically disordered and coiled-coil HMSV associate with MyoE in a stable HUM (HMSV-UDS1-MyoE) complex recruited by RAB11 to SVs through an interaction network involving RAB11 and HUM components, with the MyoE globular tail domain (GTD) binding both HMSV and RAB11-GTP and RAB11-GTP binding both the MyoE-GTD and UDS1. UDS1 bridges RAB11-GTP to HMSV, an avid interactor of the MyoE-GTD. The interaction between the UDS1-HMSV sub-complex and RAB11-GTP can be reconstituted in vitro. Ablating UDS1 or HMSV impairs actomyosin-mediated transport of SVs to the apex, resulting in spreading of RAB11 SVs across the apical dome as KinA/microtubule-dependent transport gains prominence.

The Endoplasmic Reticulum Cargo Receptor FgErv14 Regulates DON Production, Growth and Virulence in Fusarium graminearum

Fusarium graminearum is a plant filamentous pathogenic fungi and the predominant causal agent of Fusarium head blight (FHB) in cereals worldwide. The regulators of the secretory pathway contribute significantly to fungal mycotoxin synthesis, development, and virulence. However, their roles in these processes in F. graminearum remain poorly understood. Here, we identified and functionally characterized the endoplasmic reticulum (ER) cargo receptor FgErv14 in F. graminearum. Firstly, it was observed that FgErv14 is mainly localized in the ER. Then, we constructed the FgErv14 deletion mutant (ΔFgerv14) and found that the absence of the FgErv14 caused a serious reduction in vegetative growth, significant defects in asexual and sexual reproduction, and severely impaired virulence. Furthermore, we found that the ΔFgerv14 mutant exhibited a reduced expression of TRI genes and defective toxisome generation, both of which are critical for deoxynivalenol (DON) biosynthesis. Importantly, we found the green fluorescent protein (GFP)-tagged FgRud3 was dispersed in the cytoplasm, whereas GFP-FgSnc1-PEM was partially trapped in the late Golgi in ΔFgerv14 mutant. These results demonstrate that FgErv14 mediates anterograde ER-to-Golgi transport as well as late secretory Golgi-to-Plasma membrane transport and is necessary for DON biosynthesis, asexual and sexual reproduction, vegetative growth, and pathogenicity in F. graminearum.

Comment on Dimou et al. Profile of Membrane Cargo Trafficking Proteins and Transporters Expressed under N Source Derepressing Conditions in Aspergillus nidulans. J. Fungi 2021, 7, 560

Contrary to the opinion recently offered by Dimou et al., our previously published biochemical, subcellular and genetic data supported our contention that AN11127 corresponds to the A. nidulans gene encoding Sec12, which is the guanine nucleotide exchange factor (GEF) specific for SAR1. We add here additional bioinformatics evidence that fully disprove the otherwise negative evidence reported by Dimou et al., highlighting the dangers associated with the lax interpretation of genomic data. On the positive side, we establish guidelines for the identification of this key secretory gene in other species of Ascomycota and Basidiomycota, including species of medical and applied interest.

The fungal RABOME: RAB GTPases acting in the endocytic and exocytic pathways of Aspergillus nidulans (with excursions to other filamentous fungi)

RAB GTPases are major determinants of membrane identity that have been exploited as highly specific reporters to study intracellular traffic in vivo. A score of fungal papers have considered individual RABs, but systematic, integrated studies on the localization and physiological role of these regulators and their effectors have been performed only with Aspergillus nidulans. These studies have influenced the intracellular trafficking field beyond fungal specialists, leading to findings such as the maturation of trans-Golgi (TGN) cisternae into post-Golgi RAB11 secretory vesicles, the concept that these RAB11 secretory carriers are loaded with three molecular nanomotors, the understanding of the role of endocytic recycling mediated by RAB6 and RAB11 in determining the hyphal mode of life, the discovery that early endosome maturation and the ESCRT pathway are essential, the identification of specific adaptors of dynein-dynactin to RAB5 endosomes, the exquisite dependence that autophagy displays on RAB1 activity, the role of TRAPPII as a GEF for RAB11, or the conclusion that the RAB1-to-RAB11 transition is not mediated by TRAPP maturation. A remarkable finding was that the A. nidulans Spitzenkörper contains four RABs: RAB11, Sec4, RAB6, and RAB1. How these RABs cooperate during exocytosis represents an as yet outstanding question.

En bloc TGN recruitment of Aspergillus TRAPPII reveals TRAPP maturation as unlikely to drive RAB1-to-RAB11 transition

Transport protein particle (TRAPP) complexes regulate membrane traffic. TRAPPII and TRAPPIII share a core hetero-heptamer, also denoted TRAPPI. In fungi TRAPPIII and TRAPPII mediate GDP exchange on RAB1 and RAB11, respectively, regulating traffic across the Golgi, with TRAPPIII also activating RAB1 in autophagosomes. Our finding that Aspergillus nidulans TRAPPII can be assembled by addition of a TRAPPII-specific subcomplex onto core TRAPP prompted us to investigate the possibility that TRAPPI and/or TRAPPIII already residing in the Golgi matures into TRAPPII to determine a RAB1-to-RAB11 conversion as Golgi cisternae progress from early Golgi to TGN identity. By time-resolved microscopy, we determine that the TRAPPII reporter Trs120 (the homolog of metazoan TRAPPC9) is recruited to existing trans-Golgi network (TGN) cisternae slightly before RAB11 arrives, and resides for ∼45 s on them before cisternae tear off into RAB11 secretory carriers. Notably, the core TRAPP reporter Bet3 (the homolog of metazoan TRAPPC3) was not detectable in early Golgi cisternae, being instead recruited to TGN cisternae simultaneously with Trs120, indicating en bloc recruitment of TRAPPII to the Golgi and arguing strongly against the TRAPP maturation model.

System-Wide Characterization of MoArf GTPase Family Proteins and Adaptor Protein MoGga1 Involved in the Development and Pathogenicity of Magnaporthe oryzae

ADP ribosylation factor (Arf) small GTPase family members are involved in vesicle trafficking and organelle maintenance in organisms ranging from Saccharomyces cerevisiae to humans. A previous study identified Magnaporthe oryzae Arf6 (MoArf6) as one of the Arf proteins that regulates growth and conidiation in the rice blast fungus M. oryzae, but the remaining family proteins remain unknown. Here, we identified six additional Arf proteins, including MoArf1, MoArl1, MoArl3, MoArl8, MoCin4, and MoSar1, as well as their sole adaptor protein, MoGga1, and determined their shared and specific functions. We showed that the majority of these proteins exhibit positive regulatory functions, most notably, in growth. Importantly, MoArl1, MoCin4, and MoGga1 are involved in pathogenicity through the regulation of host penetration and invasive hyphal growth. MoArl1 and MoCin4 also regulate normal vesicle trafficking, and MoCin4 further controls the formation of the biotrophic interfacial complex (BIC). Moreover, we showed that Golgi-cytoplasm cycling of MoArl1 is required for its function. Finally, we demonstrated that interactions between MoArf1 and MoArl1 with MoGga1 are important for Golgi localization and pathogenicity. Collectively, our findings revealed the shared and specific functions of Arf family members in M. oryzae and shed light on how these proteins function through conserved mechanisms to govern growth, transport, and virulence of the blast fungus.IMPORTANCEMagnaporthe oryzae is the causal agent of rice blast, representing the most devastating diseases of rice worldwide, which results in losses of amounts of rice that could feed more than 60 million people each year. Arf (ADP ribosylation factor) small GTPase family proteins are involved in vesicle trafficking and organelle maintenance in eukaryotic cells. To investigate the function of Arf family proteins in M. oryzae, we systematically characterized all seven Arf proteins and found that they have shared and specific functions in governing the growth, development, and pathogenicity of the blast fungus. We have also identified the pathogenicity-related protein MoGga1 as the common adaptor of MoArf1 and MoArl1. Our findings are important because they provide the first comprehensive characterization of the Arf GTPase family proteins and their adaptor protein MoGga1 functioning in a plant-pathogenic fungus, which could help to reveal new fungicide targets to control this devastating disease.

Identification of the guanine nucleotide exchange factor for SAR1 in the filamentous fungal model Aspergillus nidulans

In spite of its basic and applied interest, the regulation of ER exit by filamentous fungi is insufficiently understood. In previous work we isolated a panel of conditional mutations in sarA encoding the master GTPase SarA^SAR1 in A. nidulans and demonstrated its key role in exocytosis and hyphal morphogenesis. However, the SAR1 guanine nucleotide exchange factor (GEF), Sec12, has not been characterized in any filamentous fungus, largely due to the fact that SEC12 homologues share little amino acid sequence identity beyond a GGGGxxxxGϕxN motif involved in guanine nucleotide exchange. Here we demonstrate that AN11127 encodes A. nidulans Sec12, which is an essential protein that localizes to the ER and that, when overexpressed, rescues the growth defect resulting from a hypomorphic sarA6^ts mutation at 37 °C. Using purified, bacterially expressed proteins we demonstrate that the product of AN11127 accelerates nucleotide exchange on SarA^SAR1, but not on its closely related GTPase ArfA^ARF1, as expected for a bona fide GEF. The unequivocal characterization of A. nidulans Sec12 paves the way for the tailored modification of ER exit in a model organism that is closely related to industrial species of filamentous fungi.

The phospholipid flippase DnfD localizes to late Golgi and is involved in asexual differentiation in Aspergillus nidulans

The maintenance of cell shape requires finely tuned and robust vesicle trafficking in order to provide sufficient plasma membrane materials. The hyphal cells of filamentous fungi are an extreme example of cell shape maintenance due to their ability to grow rapidly and respond to the environment while keeping a relatively consistent shape. We have previously shown that two phospholipid flippases, which regulate the asymmetry of specific phospholipids within the plasma membrane, are important for hyphal growth in Aspergillus nidulans. Here, we examine the rest of the phospholipid flippases encoded by A. nidulans by obtaining single and double deletions of all four family members, dnfA, dnfB, dnfC, and dnfD. We find that deleting dnfC does not impart a noticeable phenotype, by itself or with other deletions, but that dnfD, the homolog of the essential yeast gene neo1, is important for conidiation. dnfD deletion mutants form misshapen conidiophore vesicles that are defective in metulae formation. We localize DnfD to late Golgi equivalents, where it appears just before dissociation of this organelle. We propose that DnfD functions in a trafficking process that is specifically required for the morphological changes that take place during conidiation.

COPI localizes to the early Golgi in Aspergillus nidulans

Coatomer-I (COPI) is a heteromeric protein coat that facilitates the budding of membranous carriers mediating Golgi-to-ER and intra-Golgi transport. While the structural features of COPI have been thoroughly investigated, its physiological role is insufficiently understood. Here we exploit the amenability of A. nidulans for studying intracellular traffic, taking up previous studies by Breakspear et al. (2007) with the α-COP/CopA subunit of COPI. Endogenously tagged α-COP/CopA largely localizes to SedV^Sed5 syntaxin-containing early Golgi cisterna, and acute inactivation of ER-to-Golgi traffic delocalizes COPI to a haze, consistent with the cisternal maturation model. In contrast, the Golgi localization of COPI is independent of the TGN regulators HypB^Sec7 and HypA^Trs120, implying that COPI budding predominates at the SedV^Sed5 early Golgi, with lesser contribution of the TGN. This finding agrees with the proposed role of COPI-mediated intra-Golgi retrograde traffic in driving cisternal maturation, which predicts that the capacity of the TGN to generate COPI carriers is low. The COPI early Golgi compartments intimately associates with Sec13-containing ER exit sites. Characterization of the heat-sensitive copA1ts (sod^VIC1) mutation showed that it results in a single residue substitution in the ε-COP-binding Carboxyl-Terminal-Domain of α-COP that likely destabilizes its folding. However, we show that Golgi disorganization by copA1ts necessitates >150 min-long incubation at 42 °C. This weak subcellular phenotype makes it unsuitable for inactivating COPI traffic acutely for microscopy studies, and explains the aneuploidy-stabilizing role of the mutation at subrestrictive temperatures.

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.

Cell Biology of Hyphal Growth

Filamentous fungi are a large and ancient clade of microorganisms that occupy a broad range of ecological niches. The success of filamentous fungi is largely due to their elongate hypha, a chain of cells, separated from each other by septa. Hyphae grow by polarized exocytosis at the apex, which allows the fungus to overcome long distances and invade many substrates, including soils and host tissues. Hyphal tip growth is initiated by establishment of a growth site and the subsequent maintenance of the growth axis, with transport of growth supplies, including membranes and proteins, delivered by motors along the cytoskeleton to the hyphal apex. Among the enzymes delivered are cell wall synthases that are exocytosed for local synthesis of the extracellular cell wall. Exocytosis is opposed by endocytic uptake of soluble and membrane-bound material into the cell. The first intracellular compartment in the endocytic pathway is the early endosomes, which emerge to perform essential additional functions as spatial organizers of the hyphal cell. Individual compartments within septated hyphae can communicate with each other via septal pores, which allow passage of cytoplasm or organelles to help differentiation within the mycelium. This article introduces the reader to more detailed aspects of hyphal growth in fungi.

Aspergillus nidulans Ambient pH Signaling Does Not Require Endocytosis

Aspergillus nidulans (Pal) ambient pH signaling takes place in cortical structures containing components of the ESCRT pathway, which are hijacked by the alkaline pH-activated, ubiquitin-modified version of the arrestin-like protein PalF and taken to the plasma membrane. There, ESCRTs scaffold the assembly of dedicated Pal proteins acting downstream. The molecular details of this pathway, which results in the two-step proteolytic processing of the transcription factor PacC, have received considerable attention due to the key role that it plays in fungal pathogenicity. While current evidence strongly indicates that the pH signaling role of ESCRT complexes is limited to plasma membrane-associated structures where PacC proteolysis would take place, the localization of the PalB protease, which almost certainly catalyzes the first and only pH-regulated proteolytic step, had not been investigated. In view of ESCRT participation, this formally leaves open the possibility that PalB activation requires endocytic internalization. As endocytosis is essential for hyphal growth, nonlethal endocytic mutations are predicted to cause an incomplete block. We used a SynA internalization assay to measure the extent to which any given mutation prevents endocytosis. We show that none of the tested mutations impairing endocytosis to different degrees, including slaB1, conditionally causing a complete block, have any effect on the activation of the pathway. We further show that PalB, like PalA and PalC, localizes to cortical structures in an alkaline pH-dependent manner. Therefore, signaling through the Pal pathway does not involve endocytosis.

TRAPPII regulates exocytic Golgi exit by mediating nucleotide exchange on the Ypt31 ortholog RabERAB11

The oligomeric complex transport protein particle I (TRAPPI) mediates nucleotide exchange on the RAB GTPase RAB1/Ypt1. TRAPPII is composed of TRAPPI plus three additional subunits, Trs120, Trs130, and Trs65. Unclear is whether TRAPPII mediates nucleotide exchange on RAB1/Ypt1, RAB11/Ypt31, or both. In Aspergillus nidulans, RabO(RAB1) resides in the Golgi, RabE(RAB11) localizes to exocytic post-Golgi carriers undergoing transport to the apex, and hypA encodes Trs120. RabE(RAB11), but not RabO(RAB1), immunoprecipitates contain Trs120/Trs130/Trs65, demonstrating specific association of TRAPPII with RabE(RAB11) in vivo. hypA1(ts) rapidly shifts RabE(RAB11), but not RabO(RAB1), to the cytosol, consistent with HypA(Trs120) being specifically required for RabE(RAB11) activation. Missense mutations rescuing hypA1(ts) at 42 °C mapped to rabE, affecting seven residues. Substitutions in six, of which four resulted in 7- to 36-fold accelerated GDP release, rescued lethality associated to TRAPPII deficiency, whereas equivalent substitutions in RabO(RAB1) did not, establishing that the essential role of TRAPPII is facilitating RabE(RAB11) nucleotide exchange. In vitro, TRAPPII purified with HypA(Trs120)-S-tag accelerates nucleotide exchange on RabE(RAB11) and, paradoxically, to a lesser yet substantial extent, on RabO(RAB1). Evidence obtained by exploiting hypA1-mediated destabilization of HypA(Trs120)/HypC(Trs130)/Trs65 assembly onto the TRAPPI core indicates that these subunits sculpt a second RAB binding site on TRAPP apparently independent from that for RabO(RAB1), which would explain TRAPPII in vitro activity on two RABs. Using A. nidulans in vivo microscopy, we show that HypA(Trs120) colocalizes with RabE(RAB11), arriving at late Golgi cisternae as they dissipate into exocytic carriers. Thus, TRAPPII marks, and possibly determines, the Golgi-to-post-Golgi transition.