Absence of glycosylation on cyanobacterial phycobilisome linker polypeptides and rhodophytan phycoerythrins

Abstract

The 27-, 30-, and 33-kDa rod linker polypeptides and the 75-kDa core linker of phycobilisomes from the cyanobacterium Synechococcus sp. strain PCC 7942 have been reported to be glycoproteins with carbohydrate contents ranging from 3.2 to 18.8% and composed of N-acetylgalactosamine and glucose (H.C. Riethman, T.P. Mawhinney, and L.A. Sherman, J. Bacteriol. 170:2433-2440, 1988). Synechococcus sp. strain PCC 7942 phycobilisomes were purified extensively, and the linker polypeptides were separated from the phycobiliproteins by precipitation in 1 M NaSCN. Upon hydrolysis, the linker fraction yielded 0.037% glucose and 0.015% galactosamine by weight and no other carbohydrate. Phycobilisome polypeptides separated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate were subjected to various glycoprotein-specific staining procedures. Linker polypeptides showed very weak concanavalin A binding and no staining by the Schiff-periodate method or by a much more sensitive periodate oxidation-based method. These results indicated that the linker polypeptides are not glycosylated. An earlier report (T. Fujiwara, J. Biochem. 49:361-367, 1961) contended, on the basis of the isolation of sugar-containing peptic chromopeptides from Porphyra tenera R-phycoerythrin, that this red algal phycobiliprotein is a glycoprotein. Analysis of Gastroclonium coulteri R-phycoerythrin and Porphyridium cruentum B-phycoerythrin revealed only traces of carbohydrate in these two proteins, 0.36 and 0.14%, respectively. Results of glycoprotein staining of gels suggested that the carbohydrate in the R-phycoerythrin preparation is due to a glycoprotein contaminant and that neither red algal phycoerythrin is glycosylated.