Acute inactivation of the Aspergillus nidulans Golgi membrane fusion machinery: correlation of apical extension arrest and tip swelling with cisternal disorganization

Abstract

The mechanisms governing traffic across the Golgi are incompletely understood. We studied, by live-cell microscopy, the consequences of disorganizing the Aspergillus nidulans Golgi, using an extended set of fluorescent protein markers to resolve early from late cisternae. The early Golgi syntaxin SedV(Sed) (5) and the RabO(Rab) (1) regulatory GTPase play essential roles in secretion, cooperating in the ER-Golgi interface. Following a temperature shift-up 'on-the-stage', hyphae carrying engineered sedV(R258G) and rabO(A136D) ts mutations arrest polarized growth. This arrest correlates with overall Golgi disorganization and characteristic hyphal tip swelling. Using v-SNARE SynA as reporter, we show that the sedV(R258G) phenotypes correlate with arrested secretion. Both the morphogenetic defect and the secretory deficit are reversible. Thus downregulation of secretion, like that of endocytosis, has morphogenetic consequences, implying that mechanisms tuning the secretory pathway might be involved in developmental processes. According to the cisternal maturation model, acute impairment of traffic in the ER-Golgi interface should lead to disorganization of both the early and the late Golgi cisternae. Thus, the relatively rapid late Golgi disorganization observed upon shifting ER-Golgi interface mutants to the restrictive temperature seems incompatible with an A. nidulans Golgi network organized on the basis of stable early and late compartments, supporting instead cisternal maturation.

TRAPPII regulates exocytic Golgi exit by mediating nucleotide exchange on the Ypt31 ortholog RabE RAB11

“…In fungi, anterograde Golgi traffic is governed by RAB1 and RAB11 homologs. In Saccharomyces cerevisiae and Aspergillus nidulans, RAB1 homologs (Ypt1 and RabO, respectively) (7,8) mediate biogenesis of Golgi cisternae and anterograde traffic across them, whereas RAB11 homologs (Ypt31/Ypt32 and RabE, respectively) (9, 10) mediate Golgi exit of exocytic carriers and control their trafficking to the plasma membrane. The sharp functional boundary between Ypt1 and Ypt31/Ypt32 is determined, at least in part, by an anticurrent cascade in which Gyp1, the GAP for Ypt1, is an effector of Ypt31/Ypt32, thus ensuring minimal overlap of the GTPases on the same membranes (11).…”

Fungal Morphogenesis

“…Thus, in the Golgi, different classes of vesicles must be sorted and matched to their correct cargoes. The importance of the Golgi to vesicle trafficking and morphogenesis has long been established in S. cerevisiae (Schekman and Novick 2004), but only recently has its importance for sustained polar growth been described for filamentous fungi (Pinar et al 2013). Indeed, whereas the Spitzenkörper appears to be required primarily for rapid rates of hyphal extension, polarized growth fails completely in the absence of a functional Golgi complex.…”

F-Box Protein RcyA Controls Turnover of the Kinesin-7 Motor KipA in Aspergillus nidulans

“…Besides that, SynA and RcyA colocalized at small dots at subapical regions, which might represent the late Golgi or trans-Golgi network (Fig. 3C, arrows) (43). Because conventional kinesin (kinesin-1) is involved in protein secretion, we hypothesized that KinA could be involved in RcyA movement (44)(45)(46).…”