Transmembrane domains of the syndecan family of growth factor coreceptors display a hierarchy of homotypic and heterotypic interactions

Abstract

The single-pass transmembrane domains (TMDs) of the syndecan family of cell surface adhesion molecules have been implicated in functional protein-protein interactions. Although each paralog contains a conserved GxxxG dimerization motif, we show here that the syndecan-1 TMD dimerizes weakly, the syndecan-3 and syndecan-4 TMDs each dimerize strongly, and the syndecan-2 TMD dimerizes very strongly. These markedly different levels of selfassociation suggest that paralog TMDs play different roles in directing functional interactions of each full-length syndecan family member. We further show that each syndecan TMD forms detergent-resistant heteromeric complexes with other paralogs, and that these interactions exhibit selectivity. Although heteromeric interactions among full-length syndecan paralogs have not been reported, we argue that the distinct hierarchy of proteinprotein interactions mediated by the syndecan TMDs may give rise to considerable complexity in syndecan function. The demonstration that TMD homodimerization and heterodimerization can be mediated by GxxxG motifs and modulated by sequence context has implications for the signaling mechanisms of other cell surface receptors, including the integrins and the erbB family.homodimerization ͉ heterodimerization ͉ selectivity S yndecans are cell surface receptors that participate in cellcell and cell-matrix interactions critical to animal development. In mammals, where four syndecan paralogs exist (1-5), syndecan expression is tissue-specific and developmentally regulated, with most cells expressing one or more syndecans (6, 7). Syndecan-1 plays roles in early development (8-10) and wound healing (11), and syndecan-2 participates in neuronal dendritic spine morphogenesis (12), angiogenic sprouting (13), and Xenopus left/right axis formation (14). Syndecan-3 is involved in skeletogenesis (15), and syndecan-4 participates in the formation of integrin-containing focal adhesions (16). Although our knowledge of the mechanisms underlying syndecan biology is still sparse, it is already clear that understanding syndecan function has implications for treatment of cancer, wound healing, and viral and bacterial pathogenesis (reviewed in ref. 17).Each syndecan contains an ectodomain, a transmembrane domain (TMD), and a short, C-terminal cytoplasmic domain. Syndecan family members interact with a wide array of partners: the divergent heparan sulfate-modified ectodomains bind matrix proteins and growth factors, and the cytosolic domains bind cytoskeletal proteins and PDZ-domain proteins through the C1 and C2 regions (reviewed in refs. 18 and 19). Syndecan TMDs are also thought to make functional interactions: some functions of syndecan-2 and syndecan-4 depend on self-association through their TMDs (20), and the TMD of syndecan-1 makes interactions that lead to the initial spreading response of Raji cells (21). The first evidence of TMD involvement in syndecan protein-protein interactions was the attribution of detergentresistant noncovalent oligomerization of the syndecan-3 ...

A Unique Phenylalanine in the Transmembrane Domain Strengthens Homodimerization of the Syndecan-2 Transmembrane Domain and Functionally Regulates Syndecan-2

“…Although the TMD is known to be involved in regulating receptor dimerization/oligomerization, we know little about the factors that control the dimerization/oligomerization of TMDs. Syndecans are known to self-associate through their conserved TMDs (13,14). Furthermore, a conserved GXXXG motif in the TMD is thought to enable the formation of stable TMD dimers, which is crucial for the functions of syndecan-2 and -4 (13).…”

“…Interestingly, a previous study using synthetic peptides showed that the syndecan TMDs have different dimerization affinities (14). This suggests that other residues of the syndecan TMD, beyond the GXXXG motif, might participate in the TMD-TMD association and related syndecan functions.…”

“…All syndecans contain GXXXG motifs and form SDS-resistant homodimers (13,14). The TMD is sufficient for syndecan dimerization, and experiments using a syndecan mutant in which the glycines of the GXXXG motif were mutated to leucines showed that syndecan dimerization is mediated by this motif (13).…”

Trans-regulation of Syndecan Functions by Hetero-oligomerization

“…Given the high pairwise sequence identity of syndecan transmembrane domains (48 -72%) and the conservation of the GXXXG dimerization motif, all syndecan transmembrane domains might be expected to mediate heteromeric oligomerization as well as homomeric interactions. A previous study using synthetic peptides corresponding to the transmem-brane domain of each syndecan paralog has measured the association tendencies of the syndecan transmembrane domain (11). This study found that the transmembrane domains of the four syndecan paralogs self-associate to very different degrees and are capable of a distinct set of heteromeric interactions, independent of extracellular and cytoplasmic domains (11).…”

“…A previous study using synthetic peptides corresponding to the transmem-brane domain of each syndecan paralog has measured the association tendencies of the syndecan transmembrane domain (11). This study found that the transmembrane domains of the four syndecan paralogs self-associate to very different degrees and are capable of a distinct set of heteromeric interactions, independent of extracellular and cytoplasmic domains (11). However, whether full-length syndecans form heterodimers and, if so, how this heterodimerization is involved in regulating syndecan functions, have not been investigated.…”