Acute inactivation of the A spergillus nidulans G olgi membrane fusion machinery: correlation of apical extension arrest and tip swelling with cisternal disorganization

Summary The mechanisms governing traffic across the G olgi are incompletely understood. We studied, by live‐cell microscopy, the consequences of disorganizing the A spergillus nidulans   G olgi, using an extended set of fluorescent protein markers to resolve early from late cisternae. The early G olgi syntaxin SedV Sed 5 and the RabO Rab 1 regulatory GTPase play essential roles in secretion, cooperating in the ER – G olgi 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 G olgi 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 – G olgi interface should lead to disorganization of both the early and the late G olgi cisternae. Thus, the relatively rapid late G olgi disorganization observed upon shifting ER – G olgi interface mutants to the restrictive temperature seems incompatible with an A . nidulans   G olgi network organized on the basis of stable early and late compartments, supporting instead cisternal maturation.