Pir proteins of Saccharomyces cerevisiae are attached to β-1,3-glucan by a new protein-carbohydrate linkage

A family of covalently linked cell wall proteins of Saccharomyces cerevisiae, called Pir proteins, are characterized by up to 10 conserved repeating units. Ccw5/Pir4p contains only one complete repeating sequence and its deletion caused a release of the protein into the medium. The exchange of each of three glutamines ( Gln(69), Gln(74), Gln(76)) as well as one aspartic acid ( Asp(72)) within the repeating unit leads to a loss of the protein from the cell wall. Amino acid sequencing revealed that only Gln(74) is modified. Release of the protein with mild alkali, changed Gln(74) to glutamic acid, suggesting that Gln(74) is involved in the linkage. Analysis by mass spectrometry showed that 5 hexoses are attached to Gln/Glu(74). Sugar analysis revealed glucose as the only constituent. It is suggested that Pir proteins form novel, alkali labile ester linkages between the gamma-carboxyl group of glutamic acids, arising from specific glutamines, with hydroxyl groups of glucoses of beta-1,3-glucan chains. This transglutaminase-type reaction could take place extracellularly and would energetically proceed on the account of amido group elimination.