Isolation of the cDNA encoding glycoprotein-2 (GP-2), the major zymogen granule membrane protein. Homology to uromodulin/Tamm-Horsfall protein

Abstract

GP-2, a 78-kDa glycoprotein, is the major component of zymogen granule membranes of the exocrine pancreas. We report the isolation of the cDNA encoding for GP-2 from a rat pancreatic lambda gt-11 cDNA library. The cDNA is 1921 base pairs in length with an open reading frame encoding for a protein of 530 amino acids containing eight potential N-glycosylation sites. It encompasses the amino terminus of the protein including the signal sequence as evidenced by in vitro transcription/translation experiments conducted in the presence of dog pancreas rough microsomes in which the protein underwent apparent core glycosylation. A hydrophobic stretch of amino acids is present at the carboxyl terminus which is likely to serve as a signal for attachment of a glycosyl phosphatidylinositol (GPI) membrane anchor as has been described for GP-2. When the cDNA was introduced into HeLa cells by transfection, the expressed protein was located on the cell surface and could be released by incubation of the cells with phosphatidylinositol-specific phospholipase C, confirming that it is GPI-linked. Upon searching through the GenBank database, the GP-2 amino acid sequence was found to have a 53% identity and 85% similarity to human uromodulin/Tamm-Horsfall protein (THP) over a 450-amino acid stretch that encompassed all 28 cysteines after the signal sequence of GP-2. Uromodulin/Tamm-Horsfall protein, an 85-kDa glycoprotein synthesized by the kidney, shares several characteristics with GP-2 in addition to its sequence similarity. Both are attached to the membrane by a GPI anchor, but are also released from the apical surface of their respective cells and subsequently form large aggregates. Together they may define a new gene family.