Localization and Characterization of an Integral Membrane Protein Antigen Expressed by Pigment Cells in Embryos of the Sea Urchin Strongylocentrotus purpuratus

Chromogenic mesenchymal cells in plutei of the sea urchin Strongylocentrotus purpuratus express a tissue specific epitope recognized by the monoclonal antibody Sp–1. In transmission electron micrographs of pre-embed immunoperoxidase labelled plutei, the epitope is localized to pigment cell surfaces. Cell membranes of epidermal cells apposed to pigment cells are also immunoreactive, but endodermal cells apposed to pigment cells are not. Separation of Sp–1 immunoreactive material into the aqueous phase after embryo extraction in butanol indicates that the antigen is a protein or glycoprotein, and other solubilization characteristics suggest that it is an integral membrane constituent. The epitope is destroyed by general proteases and treatment with guanidine hydrochloride, and is resistant to oxidation by periodate and glycosyloxidases, suggesting that it is peptide rather than carbohydrate. On immunoblots of whole embryo extracts, or after SDS-PAGE analysis of ³⁵S-methionine-labelled embryos immunoprecipitated with Sp–1, a band showing an apparent molecular mass of 110 kD is seen at all stages from mid-gastrula to 26-day pluteus. We conclude that the Sp–1 antigen is a 110 kD integral membrane protein which may interact with the epidermal cells over which the pigment cells migrate.