THE CYTOPLASMIC DOMAIN OF SYNDECAN-1 IS NOT REQUIRED FOR ASSOCIATION WITH TRITON X-100-INSOLUBLE MATERIAL

Cell surface heparan sulfate proteoglycans such as syndecan-1 bind var ious extracellular matrix proteins and have been suggested to interact with the cytoskeleton. Such interactions are thought to be important for stabilizing cell morphology. Syndecan-1 resists extraction with Tr iton X-100. This insolubility was reported not to be affected by remov al of the glycosaminoglycan chains, suggesting that the insolubility i s not due to binding to the extracellular matrix, but rather to an ass ociation with the actin cytoskeleton (Rapraeger, A., Jalkanen, M. and Bernfield, M. (1986) J. Cell Biol. 103, 2683-2696). To examine further the interaction of syndecan-1 with the Triton X-100-insoluble residue , we expressed wild-type mouse syndecan-1 and a cytoplasmic deletion m utant (tail-less) in Chinese hamster ovary cells. We observed that bot h the wild-type and the tail-less syndecan-1 were partly insoluble in Triton X-100. The insolubility was not affected by increasing temperat ure (37 degrees C or 50 degrees C) or by cytochalasin D. Removal of th e glycosaminoglycan chains from the ectodomain, however, resulted in c omplete Triton X-100 solubility, unlike previous reports, Syndecan-1 c ould also be released into the Triton X-100-soluble fraction by additi on of heparin or heparan sulfate to the extraction medium. We conclude that the cytoplasmic domain of syndecan-1 is not responsible for Trit on X-100 insolubility. Instead, our results indicate that Triton X-100 insolubility is caused by an interaction of syndecan-1 molecules with other cellular and/or extracellular molecules mediated by the heparan sulfate chains.