Comparison of expression patterns and cell adhesion properties of the mouse biliary glycoproteins Bgp1 and Bgp2

Biliary glycoproteins are members of the carcinoembryonic antigen (CEA) fam ily and behave as cell adhesion molecules. The mouse genome contains two ve ry similar Bgp genes, Bgp1 and Bgp2, whereas the human and rat genomes cont ain only one BGP gene. A Bgp2 isoform was previously identified as an alter native receptor for the mouse coronavirus mouse hepatitis virus. This isofo rm consists of two extracellular immunoglobulin domains, a transmembrane do main and a cytoplasmic tail of five amino acids. In this report, we have ex amined whether the Bgp2 gene can express other isoforms in different mouse tissues. We found only one other isoform, which has a long cytoplasmic tail of 73 amino acids. The long cytodomain of the Bgp2 protein is highly simil ar to that of the Bgp1/4L isoform. The Bgp2 protein is expressed in low amo unts in kidney and in a rectal carcinoma cell line. Antibodies specific to Bgp2 detected a 42-kDa protein, which is expressed at the cell surface of t hese samples. Bgp2 was found by immunocytochemistry in smooth muscle layers of the kidney, the uterus, in gut mononuclear cells and in the crypt epith elia of intestinal tissues. Transfection studies showed that, in contrast w ith Bgp1. the Bgp2 glycoprotein was not directly involved in intercellular adhesion. However, this protein is found in the proliferative compartment o f the intestinal crypts and in cells involved in immune recognition. This s uggests that the Bgp2 protein represents a distinctive member of the CEA fa mily; its unusual expression patterns in mouse tissues and the unique funct ions it may be fulfilling may provide novel clues about the multiple functi ons mediated by a common BGP protein in humans and rats.