Human C3b/C4b Receptor (Cr1): Isolation, Protein Sequence Analysis, and Cloning of a Partial cDNA from Human Tonsil

Abstract

The human receptor for the C3b/C4b fragments of complement, designated CR1, is a large glycoprotein comprised of a single polypeptide chain that exhibits genetically regulated structural and quantitative polymorphisms. The structural polymorphisms are manifested in an autosomal codominant expression of different allotypic proteins; all allotypic forms are capable of binding C3b^1,2,3,4. The most common proteins have MW of 250,000 (F allotype) and 260,000 (S allotype) by NaDodSO/PAGE, although rarer forms differ by as much as 90,000 daltons⁴. Removal of the N-linked oligosaccharides does not eliminate this polymorphism, suggesting that the primary structures of the various allotypic forms are different¹. CR1 is an integral membrane glycoprotein residing on erythrocytes, polymorphonuclear leukocytes, monocytes/macrophages,fiall B and some T lymphocytes, and renal glomerular epithelial cells^5,6. Quantitative polymorphism refers to the number of receptor sites on erythrocytes, which varies by 10-fold among different individuals. Population and family studies have shown that there are two autosomal codominant alleles that determine the quantitative polymorphism.