Interaction between SPARC and tubulin in Xenopus.

Secreted protein, acidic, rich in cysteine (SPARC) is an ancient calcium-binding glycoprotein associated with the extracellular matrices of invertebrates and vertebrates. We have previously reported an intracellular association of SPARC with the 9+2 microtubule arrays of cilia on the surface ectoderm of Xenopus embryos. During early development in Xenopus, ciliated cell precursors are associated with the inner sensorial layer of the two-layered embryonic skin. The ciliated cell precursors migrate to the overlying surface ectoderm where they undergo ciliogenesis. Whole-mount immunohistochemical data indicate SPARC is associated with the ciliary tuffts until ciliated cells begin to disappear from the surface ectoderm during late tailbud development. We now report an association between SPARC and tubulin in Xenopus embryonic cell lysates by co-immunoprecipitation. Tubulin is not co-immunoprecipitated by anti-SPARC antibodies that show no cross-reactivity to Xenopus SPARC by whole-mount immunocytochemical analysis. An association of SPARC with tubulin has also been observed in pull-down assays with biotinylated SPARC as bait. These data indicate that SPARC may have intracellular and extracellular functions during development in Xenopus.