The Alkaline Lipase of the Glyoxysomal Membrane Is a Glycoprotein

Abstract

The presence of N-linked, high-mannose oligosaccharide on alkaline lipase has been demonstrated. Alkaline lipase in glyoxysomes of the endosperm of castor bean (Ricinus communiscv Hale) seedlings, is an intrinsic membrane protein, M_r62,000. The presence of glucosamine-containing oligosaccharides was shown directly by immunoprecipitat ion of lipase which had been labeledin vivoby¹⁴C-glucosamine. Lipase was eluted from concanavalin A-Sepharose at high concentration of α-methylglucoside indicating the presence of high mannose-type oligosaccharide. Two cross- reacting polypeptides (CRPs) in the glyoxysomal membrane were also identified. The two glyoxysomal CRPs, 67 kD and 86 kD, did not bind the lectin column under non-denaturing conditions. Nevertheless, the CRPs which were also abundant in carbonate-stripped endoplasmic reticulum were intensely labeled by¹⁴C-glucosamine. Treatment with endo-β-N-acetylglucosaminidase H confirmed the presence of N-linked, high mannose oligosaccharide. The possible relationship of the cross-reacting glycopolypeptides to the lipase is under study.