The nucleotide sequence of a third cyclophilin‐homologous gene from Saccharomyces cerevisiae

The nucleotide sequence of a 1558 bp DNA fragment from the right arm of chromosome III of Saccharomyces cerevisiae contains an open reading frame of 954 nucleotides with coding potential for a protein with high similarity to the ubiquitous cyclophilins which are both peptidyl‐prolyl cis ‐ trans isomerases and cyclosporin A‐binding proteins. It should, therefore, represent the third gene ( SCC3 ) of this kind from S. cerevisiae . SCC3 is present in a single copy in the genome of S. cerevisiae and results in a constitutively expressed 1·2 kb transcript during cell growth. Its putative protein product (Scc3) contains two hydrophobic cores, one at the amino terminal, 20 amino acids long, which could serve as a signal peptide, and the other one at the carboxyl end with a structure similar to a transmembrane helix. These findings suggest that Scc3 could be a secretory or, more likely, a transmembrane protein. The only cyclophilin with similar structure to that of Scc3 is ninaA from Drosophila melanogaster , a transmembrane protein which seems to be implicated in the correct folding and/or intercalation of rhodopsin in the endoplasmic reticulum of the fly photoreceptors (Stamnes, M. A. et al. , Cell 65 , 219–227, 1991). In addition, the amino and the carboxy regions of Scc3 and ninaA share a significant level of homology, which suggests that they have a similar function, albeit for different target proteins.