O-linked glycosylation of rat renal gamma-glutamyltranspeptidase adjacent to its membrane anchor domain

Abstract

Large domains rich in serine and threonine, that are likely to exhibit clusters of O-linked oligosaccharides, have been reported adjacent to the anchor of several cell surface proteins. No such domain is evident in the primary sequence of rat renal gamma-glutamyltranspeptidase. However, papain treatment of the amphipathic enzyme (Triton-purified gamma-glutamyltranspeptidase, T gamma GT), pretreated with galactose oxidase and NaB3H4 (Frielle, T., and Curthoys, N. P. (1983) Biochemistry 22, 5709-5714), yields the hydrophilic enzyme (papain-treated Triton-purified gamma-glutamyltranspeptidase, PT gamma GT) and a labeled peptide which contains both the amino-terminal membrane anchor and the sequence Pro27-Thr28-Thr29-Ser30. Since [3H]galactose was identified in this peptide, the presence of O-linked oligosaccharides was investigated. Carbohydrate analysis is consistent with the presence of two simple O-linked oligosaccharides on T gamma GT and one on PT gamma GT. Lectin blot analysis of T gamma GT and PT gamma GT was carried out after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The small subunits of both T gamma GT and PT gamma GT and the large amphipathic subunit of T gamma GT all react with the peanut agglutinin lectin, but the large subunit of PT gamma GT exhibits no such reactivity. The reactivity with PNA is consistent with the presence of one oligosaccharide with the structure galactose beta 1-3N-acetylgalactosamine alpha 1-Ser/Thr attached to each subunit of T gamma GT. The papain-sensitivity of the oligosaccharide from the larger subunit is consistent with O-glycosylation at the Thr28-Thr29-Ser30 sequence. The results of lectin blot analysis with wheat germ agglutinin imply that the content of N-linked oligosaccharides is unaffected by papain treatment of the transpeptidase. These data represent the first direct evidence for O-glycosylation of a microvillar hydrolase at a site immediately adjacent to the membrane anchor and indicates that even small clusters of Thr and Ser can be O-glycosylated. Isolated O-linked oligosaccharides may have functional significance since single Ser and Thr residues are consistently found near the membrane anchor of many cell surface proteins.