PHOSPHORYLATION OF THE HUMAN-TRANSFORMING-GROWTH-FACTOR-BETA-BINDING PROTEIN ENDOGLIN

Endoglin is an homodimeric membrane antigen with capacity to bind tran sforming growth factor-beta (TGF-beta). Phosphorylation of human endog lin was demonstrated in endothelial cells as well as in mouse fibrobla st transfectants expressing two isoforms, L-endoglin or S-endoglin, wi th distinct cytoplasmic domains. The extent of L-endoglin phosphorylat ion was found to be 8-fold higher than that of S-endoglin, and phospho peptide analyses revealed at least three different phosphorylation sit es for L-endoglin, whereas S-endoglin produces only one phosphopeptide . The immunoprecipitated L-endoglin was found to be phosphorylated mai nly on serine, and, to a minor extent, on threonine, residues. Treatme nt of the eels with TGF-beta 1 or the protein kinase C inhibitor H-7 r esulted in a reduction of the levels of endoglin phosphorylation.