ABSTRACT
Two features of syndecan core protein structure connect all the members of this family, both vertebrate and invertebrate (1-3). First, all are substituted with glycosaminoglycan chains, these usually being heparan sulfate. Second, their transmembrane and cytoplasmic domains are remarkably homologous. When Drosophila and mammalian syndecans are compared, it is clear that these domains are conserved and indicate important functional roles. Within the mammals, this conservation of structure is even more apparent (Fig. 1). For example, there are no changes in amino acid sequence between human and rodent syndecan-1 transmembrane and cytoplasmic domains. The same is true for other syndecans. In contrast, other than the conservation of glycosaminoglycan substitution sites, there is distinctly less conservation of ectodomain sequence (1). Apparently, core protein function on the outer face of the membrane is restricted to substitution with glycosaminoglycans, although the topography of these carbohydrates may also be important. The ectodomains of Xenopus and mammalian syndecan-2 core proteins, for example, are equivalent in size (4), perhaps indicating that each syndecan expresses its glycosaminoglycans with a conserved relationship to the cell surface.
