STRUCTURE OF SACCHAROMYCES-CEREVISIAE ALPHA-AGGLUTININ - EVIDENCE FOR A YEAST-CELL WALL PROTEIN WITH MULTIPLE IMMUNOGLOBULIN-LIKE DOMAINS WITH ATYPICAL DISULFIDES

alpha-Agglutinin of Saccharomyces cerevisiae is a cell wall-associated protein that mediates cell interaction in mating. Although the mature protein includes about 610 residues, the NH2-terminal half of the protein is sufficient for binding to its ligand alpha-agglutinin alpha-Agglutinin(20-351), a fully active fragment of the protein, has been purified and analyzed. Circular dichroism spectroscopy, together with sequence alignments, suggest that alpha-agglutinin(20-351) consists of three immunoglobulin variable-like domains: domain I, residues 20-104; domain IL, residues 105-199; and domain ID. residues 200-326. Peptide sequencing data established the arrangement of the disulfide bonds in alpha-agglutinin(20-351). Cys(97) is disulfide-bonded to Cys(114), forming an interdomain bond between domains I and II. Cys(202) is bonded to Cys(300), in an atypical intradomain disulfide bond between the A and F strands of domain III, Cys(227) and Cys(256) have free sulfhydryls. Sequencing also showed that at least two of three potential N-glycosylation sites with sequence Asn-Xaa-Thr are glycosylated. At least one of three Asn-Xaa-Ser sequences is not glycosylated. No residues NH2-terminal to Ser(282) were O-glycosylated, whereas Ser(282), and all hydroxy amino acid residues COOH-terminal to this position were modified. Therefore O-glycosylated Ser and Thr residues cluster in the COOH-terminal region of domain Iii. and the O-glycosylation continues into a Sen/Thr-rich sequence that extends from domain III to the COOH-terminal of the full-length protein.