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
-1. Interactions between fibronectin and some syndecans can be modulated by the extracellular matrix
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
The ectodomains show the least amount of amino acid
, 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).