Multimerin, a Platelet and Endothelial Cell Protein

Abstract

<p>p-155 is a soluble platelet protein, with a reduced subunit mobility of 155 kDa, that was first identified using a monoclonal antibody raised against platelets. This thesis describes the identification and characterization of p-155, including its structure, biosynthesis, cells of origin, and cDNA sequence. Studies of the native p-155 protein indicated that it is comprised of variably sized, disulfide linked multimers of p-155 subunits, ranging in size from a 400 kDa trimer to large multimers, many millions of daltons in size. Based on its massive, multimeric structure, the native p-155 protein was designated as multimerin. Comparisons with other multimeric proteins found in platelets indicated that multimerin is a novel protein and also one of the largest proteins stored in platelets. In addition to platelets, multimerin was also found in endothelial cells. Biosynthetic metabolic labeling studies indicated that multimerin is synthesized by Dami cells (a megakaryocytic cell line), and by endothelial cells. Multimerin is a highly glycosylated protein with complex, N-linked carbohydrate accounting for 1/3 of its molecular mass. Cleveland mapping studies were used to investigate the relationship between the different sized multimerin subunits found in platelets and Dami cells. These studies demonstrated peptide homology, indicating that p-155 and p-170 (a larger but less abundant multimerin subunit found in platelets) originate from p-196, the multimerin precursor protein identified in metabolic labeling studies.</p> <p>Multimerin antibodies were used to screen expression human endothelial cell libraries for multimerin cDNA clones and the most 5' cDNA clone was used to rescreen the library for complete 5' sequence. The complete cDNA sequence for multimerin was then determined. The multimerin cDNA sequence encodes a hydrophilic protein of 1228 amino acids with RGDS, EGF-like, partial EGF-like, and putative coiled-coil domains. In addition, the C-terminal region of multimerin resembles the globular head domain of complement Clq and collagens type VIII and X. These studies establish multimerin as a unique, multimeric platelet and endothelial cell protein. The massive size, GDS motif, and repeating structure of multimerin suggest a possible role for this protein in adhesion.</p>