Cloning and analysis of an Aspergillus nidulans Sec7 domain coding gene

Abstract

This study aimed to identify the genetic basis of the Aspergillus nidulans hypB5 mutant phenotype. A. nidulans is a filamentous fungus that is widely used as a cell biological and molecular genetic model system. Its hyphae grow by localized polar secretion, producing tubular cells. A. nidulans hypercellular strains define five unlinked genes, hypA1-hypE2, which cause hyphal morphogenesis defects at 42°C. hypA is orthologous to Saccharomyces TRS120, which mediates Golgi transit and is widely conserved. The hypB5 restrictive phenotype resembles hypA1: wide hyphae, short basal cells and small nuclei. Like hypA1, shifting hypB5 mutants from 28°C to 42°C causes cessation of tip growth but isotropic expansion of basal cells. A hypA1, hypB5 double mutant was impaired for growth at 28°C, suggesting these genes have related roles, but neither was epistatic at 37°C so they function in different pathways. The A. nidulans pRG3-AMA1 genomic library was used to clone hypB5 complementing DNA by phenotype rescue, and subcloned to a 5 kb KpnI fragment, pYY2. pYY2 was disrupted and sequenced by Tn1000 insertional mutagenesis. The pYY2 sequence is 4975 bp and encodes a putative Sec7 domain which has 81% identity to the Saccharomyces SEC7 domain. The Sec7 domain is highly conserved from yeasts to mammals. Saccharomyces SEC7 encodes a guanine nucleotide exchange factor involved in COPI vesicle formation and Golgi biogenesis. Insertions in the pYY2 non-Sec7 domain coding region complemented hypB5 efficiently, whereas those in the Sec7 domain did not, indicating that the Sec7 domain is sufficient for function. A point mutation was found in the hypB5 strain Sec7 domain, which could explain temperature sensitivity. However, the pYY2 sequence is found on chromosome I whereas hypB maps to chromosome VII. Although the origin and functional role of the point mutation in the hypB5 strain Sec7 protein remains unresolved, it appears that pYY2 contains an extragenic suppressor. Thus hypB likely encodes an element in the COPI vesicle assembly pathway.