Identification and characterization of a family of secretion-related smallGTPase-encoding genes from the filamentous fungus Aspergillus niger: a putative SEC4 homologue is not essential for growth

DNA fragments containing genetic information for five secretion-related sma ll GTPases of Aspergillus niger (srgA-E) were isolated and identified as me mbers of different Rab/Ypt subfamilies. This isolation and the search for s imilar sequences in fungal genomic and EST databases showed that, in contra st to Saccharomyces cerevisiae, filamentous fungi also possess homologues o f mammalian Rab2 GTPases. Multiple: transcripts with unusually long 5' and 3' untranslated regions were found for all srg genes. Their level of expres sion was independent of the type of carbon source used for growth. Although the transcripts of srgA and srgB were abundant to the same extent througho ut the cultivation, that of the other genes peaked during the early growth phase and then declined. Two genes, srgA and srgB, were characterized furth er. The protein encoded by srgA exhibited relatively low identity (58%) to its closest S. cerevisiae homologue SEC4, whereas the protein encoded by, s rgB showed 73% identity with S. cerevisiae YPT1. In contrast to other SEC4 homologues, srgA was unable to complement an S. cerevisiae sec4 mutant, and its disruption was not lethal in A. niger. SrgA mutants displayed a twofol d increase in their hyphal diameter, unusual apical branching and strongly reduced protein secretion during growth on glucose.