Characterization of chromaffin granule membrane glycoprotein IV

• Main Author: Getlawi, Fariha Mohammed
• Published: University of Edinburgh 1998
• Subjects: 571.53
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.651404

The membranes of chromaffin granules, the catecholamine storing vesicles of the adrenal medulla, contain several glycoproteins, which were initially identified simple as lectin-binding components and some of which have had functions ascribed to them subsequently. Thus glycoprotein I is dopamine β-monooxygenase, glycoproteins H, J, and K are intragranular proteinases, glycoproteins II and III have been sequenced but are still of unknown function, while glycoproteins VI and V are still uncharacterized glycoproteins of unknown function. A major component of the granule membrane is the H<SUP>+</SUP>-translocating ATPase, the largest subunit of which (116kDa) is a glycosylated transmembrane protein and a second glycosylated subunit name Ac45 (now termed M45) was also recently reported. Glycoprotein IV of bovine chromaffin granule membranes was purified by membrane fractionation with the non-ionic detergent Triton X-114, solubilization with n-octyl-β-glycoside and affinity chromatography on concanavalin A lectin-agarose, followed by electrophoresis from polyacrylamide gels. Purified glycoprotein IV was subjected to direct N-terminal amino acid sequencing, and purified glycoprotein IV was injected into rabbits in order to raise an antiserum used to characterize the protein further. Blue Native polyacrylamide gel electrophoresis was used to confirm that glycoprotein IV is a component of the membrane sector of the H<SUP>+</SUP>-ATPase, and the anti-glycoprotein IV serum recognized the same component as one directed against subunit M45 of this enzyme. Furthermore the N-terminal amino acid sequence of glycoprotein IV was identified within the M45 gene. Although this confirmed that glycoprotein IV and M45 are identical proteins, it also showed that they are derived from a larger precursor by removal of a 246-amino acid N-terminal sequence. In agreement with this, enzymatic deglycosylation of M45/glycoprotein IV indicated an apparent polypeptide molecular mass of 29kDa. Glycoprotein IV/M45 is found in the membranes from mouse pituitary AtT-20 from rat phaeochromocytoma (PC-12) and from human phaeochromocytoma but is absent in bovine kidney microsomal membranes and rat brain synptosomes.