ISOLATION AND ANALYSIS OF FUNCTIONAL HOMOLOGS OF THE SECRETION-RELATED SAR1 GENE OF SACCHAROMYCES-CEREVISIAE FROM ASPERGILLUS-NIGER AND TRICHODERMA-REESEI

The Aspergillus niger and Trichoderma reesei genes encoding the functi onal homologues of the small GTP-binding protein SAR1p, which is invol ved in the secretion pathway in Saccharomyces cerevisiae, have been cl oned and characterised. The A. niger gene (sar A) contains five intron s, whereas the T. reesei gene (sari) has only four. In both cases the first intron is at the same position as the single S. cerevisiae SARI intron. The encoded proteins show 70-80% identity to the SAR1 protein. Complementation of S. cevevisiae sari and sec12 mutants by expression vectors carrying the A. niger sar A and T. reesei sari cDNA clones co nfirmed that the cloned genes are functional homologues of the S. cere visiae SARI gene. Three mutant alleles of the A. niger sar A gene (D29 C, E109K, D29G/E109K), generated by site-directed mutagenesis, reveale d a thermosensitive dominant-negative phenotype in the presence of the wild-type sar A allele. This result contrasts with the situation in S . cerevisiae, where similar mutations have a thermosensitive phenotype . Taken together, our results indicate that the sar A gene is involved in an essential function in A. niger.