Growth-speed-correlated localization of exocyst and polarisome components in growth zones of Ashbya gossypii hyphal tips.

We use the fungus Ashbya gossypii to investigate how its polar growth machinery is organized to achieve sustained hyphal growth. In slowly elongating hyphae exocyst, cell polarity and polarisome proteins permanently localize as cortical cap at hyphal tips, thus defining the zone of secretory vesicle fusion. In tenfold faster growing hyphae, this zone is only slightly enlarged demonstrating a capacity of hyphal growth zones to increase rates of vesicle processing to reach higher speeds. Concomitant with this increase, vesicles accumulate as spheroid associated with the tip cortex, indicating that a Spitzenkörper forms in fast hyphae. We also found spheroid-like accumulations for the exocyst components AgSec3, AgSec5, AgExo70 and the polarisome components AgSpa2, AgBni1 and AgPea2 (but not AgBud6 or cell polarity factors such as AgCdc42 or AgBem1). The localization of AgSpa2, AgPea2 and AgBni1 depend on each other but only marginally on AgBud6, as concluded from a set of deletions. Our data define three conditions to achieve fast growth at hyphal tips: permanent presence of the polarity machinery in a confined cortical area, organized accumulation of vesicles and a subset of polarity components close to this area, and spatial separation of the zones of exocytosis (tip front) and endocytosis (tip rim).