A mutation in a COG4 homologue affects polarity establishment in the fungus Aspergillus nidulans

We have identified a mutation in the fungus Aspergillus nidulans , designated swoP (for swo llen cell phenotype), causing a temperature‐sensitive morphological defect during spore germination and polarized, hyphal growth. At 42C conidia swell to approximately 1.5 times the normal diameter and establish multiple points of polarity, which grow isotropically before arrestGrowth at 30C is essentially normal, though hyphae are slightly wider than wild type and exhibit a minor "steering" defect. Cells grown at 42C for up to 18 hours recover normal hyphal morphology upon transfer to 30C. In hyphae transferred from 30C to 42C, both terminal and sub‐apical compartments swell irregularly and may produce multiple buds. The growth defect of swoP is complemented by the wild type allele of AN7462, which shows strong sequence homology to COG4, a component of the Golgi tethering complex associated with retrograde transport of vesicles. Sequencing of the COG4 allele of the swoP mutant reveals a mutation at base pair 2672, introducing a stop codon at amino acid 780. Meiotic mapping produces a ca. 12% recombination frequency between the swoP and AcuK (acetate) loci, which is consistent with the chromosomal location of AN7462. Through high copy gene expression, functional connections were found between the COG4 homologue to other COG proteins, thus providing further support of COG4 involvement in polarity maintenance.