Abstract
Preparations of mannose-binding protein isolated from rat liver contain two distinct but homologous polypeptides. The complete primary structures of both of these polypeptides have been determined by sequencing of peptides derived from the proteins, isolation and sequencing of cDNAs for both proteins, and partial characterization of the gene for one of the proteins. Each polypeptide consists of three regions: (a) an NH2-terminal segment of 18-19 amino acids which is rich in cysteine and appears to be involved in the formation of interchain disulfide bonds which stabilize dimeric and trimeric forms of the protein, (b) a collagen-like domain consisting of 18-20 repeats of the sequence Gly-X-Y and containing 4-hydroxyproline residues in several of the Y positions, and (c) a COOH-terminal carbohydrate-binding domain of 148-150 amino acids. The sequences of the COOH-terminal domains are highly homologous to the sequence of the COOH-terminal carbohydrate-recognition portion of the chicken liver receptor for N-acetylglucosamine-terminated glycoproteins and the rat liver asialoglycoprotein receptor. Each protein is preceded by a cleaved, NH2-terminal signal sequence, consistent with the finding that this protein is found in serum as well as in the liver. The entire structure of the mannose-binding proteins is homologous to dog pulmonary surfactant apoprotein.
Highlights
Preparationsofmannose-bindingproteinisolated their localization within the cytoplasm or outside the cells fromrat liver contain two distinct buthomologous remains a matter of controversy [3, 4]
The formation of interchain disulfide bonds whisctha- tors isolated from rat liver and two found in human liver, as bilize dimeric and trimeric formsof the protein, (b) a well as thechicken agalactoglycoproteinreceptor, are homolcollagen-like domainconsistingof 18-20 repeats of ogousto each other, in aCOOH-terminal domain the sequence Gly-X-Y and containing 4-hydroxypro- of130-150 amino acids which contains the carbohydrateline residues in several of the Y positions, and (c) a binding activity [6,7,8]
A mannose-binding protein (MBPI) isolated from rabbit and ratliver was originally believed to be involved in uptake of mannose-containing glycoproteins by macrophages [9,10,11,12]; the fact that thisprotein is found exclusively in the parenchymal cells of the liver rules out thisfunction [13, 14]
Summary
An SDS-polyacrylamide gelof an unreduced preparation of MBP (Fig. 3, lane 4 ) reveals the presence of higher molecular weight bands with apparent molecular weights of 57,000 and 78,000 These bands appear restriction fragment probe, two categories of phage were detected. The mobility of this fragment is not significantly affected by reduction, indicating that the interchain disulfide bonds are notlocated within the carbohydrate-binding domain This suggests that they are likely to be near the NHz-terminal end of the polypeptide, a conclusion which is consistent with the sequence results presentedbelow. Since bothdisulfide-linked dimers codon 20 nucleotides upstream from the indicated initiator and trimers can be observed when the gels are run under methionine residue eliminates the possibility that the signal nonreducing conditions, at least two of the cysteines in this sequence is preceded by a further extension It is noteworthy NHz-terminal domain must be involved in disulfide bonds. Since there are four prolines in Y positions in MBP-A and five in MBP-C, a significant fraction of the potentially hydroxylated residues
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