Governing fungal polar cell extension

Polar morphogenesis is required for the function of elongated cell types like neuronal cells, pollen tubes and cells of filamentous fungi. The basal signalling components involved are highly conserved. Defects in polar cell shape can result in developmental disorders or death of the affected cell. In this work two components involved in polar growth were analysed at a molecular level in Neurospora crassa. These are LRG1, which is a member of the GTPase activating proteins (GAPs), and the germinal center kinase POD6. POD6 and LRG1 are proteins essential for hyphal tip elongation. Mutants of pod-6 and lrg-1 show phenotypic similarities to cot-1 temperature sensitive mutant in cessation of hyphal elongation and excessive hyperbranching. Genetic evidence and in vitro GTPase assays identify LRG1 as a RHO1 specific GAP. In strains expressing GFP::LRG1, the dynamic accumulation of the fusion protein as an apical cap was observed. This localisation depends on the three LIM domains of LRG1, a functional actin cytoskeleton and active growth. Similar to the localisation of COT1 and POD6, LRG1 localisation is influenced by dynein and kinesin-1 and the microtubule cytoskeleton.