Where to grow and where to go

Abstract

Filamentous fungi grow as very elongated tubular cells that extend by membrane extension and cell-wall biosynthesis. Membrane and enzyme delivery depend on secretory vesicles that travel along microtubules, accumulate in a structure called the Spitzenkörper and then move along actin cables towards the apical membrane. Whereas vesicle fusion and membrane insertion are well studied, less is known about the mechanisms with which the zones of vesicle fusion and hence the growth zones are defined. One mechanism by which polarity is established and maintained is the polar localization of cell-end marker proteins (CEMPs). They form multi-protein complexes with formin as F-actin polymerase. CEMP delivery depends on microtubules, and hence CEMPs coordinate the microtubule with the actin cytoskeleton. Actin filaments capture microtubule ends, and this positive feedback loop quickly establishes active growth sites. However, CEMP complexes are self-limiting, because fusing vesicles disturb local growth zones and Ca²⁺ influx pulses lead to F-actin disassembly. This model emerged from studies in Schizosaccharomyces pombe and Aspergillus nidulans. Surprisingly, deletion of CEMP-coding genes is not lethal. S. pombe mutants form T-shaped cells and A. nidulans germlings grow less straight. In comparison, CEMP-mutants had a strong phenotype in Arthrobotrys flagrans, a nematode-trapping fungus, which produces ring-like trapping structures. CEMP-mutants fail to form adhesive rings and instead form sticks. CEMP overexpression caused a hyperbranching phenotype. Hence, CEMPs are involved in polarity maintenance and play critical roles during modulations of polarity. Here, we are going to discuss the functions of CEMPs and their connections to other polarity determinants.