Solution structure of the dimeric cytoplasmic domain of syndecan-4

The syndecans, transmembrane proteoglycans which are involved in the organi zation of cytoskeleton and/or actin microfilaments, have important roles as cell surface receptors during cell-cell and/or cell-matrix interactions. S ince previous studies indicate that the function of the syndecan-4 cytoplas mic domain is dependent on its oligomeric status, the conformation of the s yndecan-4 cytoplasmic domain itself is important in the understanding of it s biological roles. Gel filtration results show that the syndecan-4, cytopl asmic domain (4L) itself forms a dimer stabilized by ionic interactions bet ween peptides at physiological pH, Commensurately, the NMR structures demon strate that syndecan-4L is a compact intertwined dimer with a symmetric cla mp shape in the central variable V region with a root-mean-square deviation between backbone atom coordinates of 0.95 Angstrom for residues Leu(186)-A la(195). The molecular surface of the 4L dimer is highly positively charged . In addition, no intersubunit NOEs in membrane proximal amino acid resides (Cl region) have been observed, demonstrating that the CI region is mostly unstructured in the syndecan-4L dimer, Interestingly, two parallel strands of 4L form a cavity in the center of the dimeric twist similar to our prev iously reported 4V structure. The overall topology of the central variable region within the 4L structure is very similar to that of 4V complexed with the phosphatidylinnositol 4,5-bisphosphate; however, the intersubunit inte raction mode is affected by the presence of C1 and C2 regions. Therefore, w e propose that although the 4V region in the full cytoplasmic domain has a tendency for strong peptide-peptide interaction, it may not be enough to ov ercome the repulsion of the Cl regions of syndecan-4L.