Molecular cloning of amphiglycan, a novel integral membrane heparan sulfate proteoglycan expressed by epithelial and fibroblastic cells

Abstract

We have synthesized an antisense oligonucleotide primer that matches a supposedly conserved sequence in messages for heparan sulfate proteoglycans with transmembrane orientations. With the aid of this primer we have amplified partial and selected full-length copies of a message from human lung fibroblasts that codes for a novel integral membrane heparan sulfate proteoglycan. The encoded protein is 198 amino-acids long, with discrete cytoplasmic, transmembrane, and amino-terminal extracellular domains. Except for the sequences that represent putative heparan sulfate chain attachment sites, the extracellular domain of this protein has a unique structure. The transmembrane and cytoplasmic domains, in contrast, are highly similar to the corresponding domains of fibroglycan and syndecan, the two cell surface proteoglycans that figured as models for the design of the antisense primer. This similarity includes the conservation of four tyrosine residues, one immediately in front of the stop transfer sequence and three in the cytoplasmic segment, and of the most proximal and most distal cytoplasmic sequences. The cDNA detects a single 2.6-kb message in cultured human lung fibroblasts and in a variety of human epithelial and fibroblastic cell lines. Polyclonal and monoclonal antibodies raised against the encoded peptide after expression as a beta-galactosidase fusion protein react with the 35-kD coreprotein of a cell surface heparan sulfate proteoglycan of human lung fibroblasts and decorate the surface of many cell types. We propose to name this proteoglycan "amphiglycan" (from the Greek words amphi, "around, on both sides of" and amphoo, "both") referring to its domain structure which extends on both sides of the plasmamembrane, and to its localization around cells of both epithelial and fibroblastic origin.