Cleavage Without Anchor Addition Accompanies the Processing of a Nascent Protein to Its Glycosylphosphatidylinositol-Anchored Form

Rough microsomal membranes from most mammalian cells, in the presence of a translation system, process nascent proteins with appropriate COOH-terminal signal peptides to their mature glycosylphosphatidylinositol (GPI)-linked forms. The present study, using preprominiplacental alkaline phosphatase as substrate, shows that as much as 10% of the mature product is cleaved correctly but is not linked to GPI. Some of the factors that influence the relative proportions of GPI linked to free mini-placental alkaline phosphatase are the amounts of GPI in the cells and the amino acid substituent at the ω site of the nascent protein. A mechanism for explaining cleavage both with and without GPI addition is presented, which supports a transamidase type of enzyme as the catalyst.