Syndecans

Syndecans are single transmembrane domain proteins that are thought to act as coreceptors, especially for G protein-coupled receptors. These core proteins carry three to five heparan sulfate and chondroitin sulfate chains which allow for interaction with a large variety of ligands including fibroblast growth factors, vascular endothelial growth factor, transforming growth factor-beta, fibronectin and antithrombin-1. Interactions between fibronectin and some syndecans can be modulated by the extracellular matrix protein tenascin-C.

Family members
The syndecan protein family is comprised of four members. Syndecans 1 and 3 and syndecans 2 and 4 making up separate subfamilies having arisen by gene duplication and divergent evolution from a single ancestral gene. The syndecan numbers reflect the order in which the cDNAs for each family member were cloned. All syndecans have an N-terminal signal peptide, an ectodomain, a single hydrophobic transmembrane domain, and a short C-terminal cytoplasmic domain. The ectodomains show the least amount of amino acid sequence conservation, not more than 10-20%, in contrast the transmembrane and cytoplasmic domains share around 60-70% amino acid sequence identity. The transmembrane domains contain an unusual alanine/glycine sequence motif while the cytoplasmic domain is essentially composed of two regions of conserved amino acid sequence (C1 and C2), separated by a central variable sequence of amino acids that is distinct for each family member (V).