Exon structure of a mannose-binding protein gene reflects its evolutionary relationship to the asialoglycoprotein receptor and nonfibrillar collagens.

Cloned cDNAs encoding mannose-binding proteins isolated from rat liver have been used to isolate one of the genes encoding this group of proteins. This gene, which encodes the minor form of binding protein (designated MBP-A), has been characterized by sequence analysis. The protein-coding portion of the mRNA for the MBP-A is encoded by four exons separated by three introns. The NH2-terminal, collagen-like portion of the protein is encoded by the first two exons. These exons resemble the exons found in the genes for nonfibrillar collagens in that the intron which divides them is inserted between the first two bases of a glycine codon and the exons do not have the 54- or 108-base pair lengths characteristic of fibrillar collagen genes. The carbohydrate-binding portion of MBP-A is encoded by the remaining two exons. This portion of the protein is homologous to the carbohydrate-recognition domain of the hepatic asialoglycoprotein receptor, which is encoded by four exons. It appears that the three COOH-terminal exons of the asialoglycoprotein receptor gene have been fused into a single exon in the MBP-A gene. The organization of the MBP-A gene is very similar to the arrangement of the gene encoding the highly homologous pulmonary surfactant apoprotein, although one of the intron positions is shifted by a single amino acid. The 3' end of a mannose-binding protein pseudogene has also been characterized.