ENDO16, A LARGE MULTIDOMAIN PROTEIN FOUND ON THE SURFACE AND ECM OF ENDODERMAL CELLS DURING SEA-URCHIN GASTRULATION, BINDS CALCIUM

Endo16 encodes a developmentally regulated protein restricted to cells participating in the formation of the archenteron during sea urchin g astrulation and to the stomach of the pluteus. The 4680-nt coding regi on of the Endo16 gene has been sequenced from overlapping cDNAs. Seque nce analysis revealed that Endo16 is a large multidomain protein start ing with a putative signal sequence at its amino terminus which is fol lowed by a cysteine-rich region, two potential heparin-binding regions , an acidic domain of 5 clustered repeats, an RGD cell binding motif, and a group of 12 additional acidic repeats. Immunolocalization by con focal and electron microscopy demonstrate that the Endo16 protein is i n the extracellular matrix and associated with the surface of endoderm al cells in the mid and hindgut of the archenteron. The two distinct a cidic repeat regions are similar to known calcium-binding sequences. A recombinant Endo16 protein containing both putative calcium-binding r epeat regions has been shown to bind radioactive calcium. Tryptic dige sts of gastrula stage protein extracts in the presence and the absence of calcium have established that calcium stabilizes Endo16 protein ag ainst proteolytic degradation. (C) 1994 Academic Press, Inc.