CLONING AND CHARACTERIZATION OF KLUYVEROMYCES-LACTIS SEC14, A GENE WHOSE PRODUCT STIMULATES GOLGI SECRETORY FUNCTION IN SACCHAROMYCES-CEREVISIAE

The Saccharomyces cerevisiae SEC14 gene encodes a cytosolic factor that is required for secretory protein movement from the Golgi complex. That some conservation of SEC14p function may exist was initially suggested by experiments that revealed immunoreactive polypeptides in cell extracts of the divergent yeasts Kluyveromyces lactis and Schizosaccharomyces pombe. We have cloned and characterized the K. lactis SEC14 gene (SEC14(KL)). Immunoprecipitation experiments indicated that SEC14(KL) encoded the K. lactis structural homolog of SEC14p. In agreement with those results, nucleotide sequence analysis of SEC14(KL) revealed a gene product of 301 residues (M(r) 34,615) and 77% identity to SEC14p. Moreover, a single ectopic copy of SEC14(KL) was sufficient to render S. cerevisiae sec14-1(Ts) mutants, or otherwise inviable sec14-129::HIS3 mutant strains, completely proficient for secretory pathway function by the criteria of growth, invertase secretion, and kinetics of vacuolar protein localization. This efficient complementation of sec14-129::HIS3 was observed to occur when the rates of SEC14p(KL) and SEC14p synthesis were reduced by a factor of 7 to 10 with respect to the wild-type rate of SEC14p synthesis. Taken together, these data provide evidence that the high level of structural conservation between SEC14p and SEC14p(KL) reflects a functional identity between these polypeptides as well. On the basis of the SEC14p and SEC14p(KL) primary sequence homology to the human retinaldehyde-binding protein, we suggest that the general function of these SEC14p species may be to regulate the delivery of a hydrophobic ligand to Golgi membranes so that biosynthetic secretory traffic can be supported.