Turnover and orientation of the major neural retina cell surface protein protected from tryptic cleavage by calcium.

Calcium protects a limited number of embryonic chick neural retina cell surface proteins from tryptic cleavage. One glycoprotein of Mr approximately 1.3 X 10(5) and pI approximately 4.8 (gp130) is present in intact retinas and, in the presence of Ca2+, is both resistant to tryptic cleavage and poorly iodinated. When cultured in vitro, iodinated retinas release into the medium a number of iodinated polypeptides; one of the major iodinated components is a polypeptide of Mr approximately 9 X 10(4) and pI approximately 4.8 (gp90). This component is also resistant to tryptic cleavage in the presence of Ca2+. Two-dimensional peptide maps of gp130 and gp90 derived respectively from iodinated retinas and their conditioned media are very similar. Maps of samples reiodinated following denaturation show the same similarities as well as additional labeled peptides. Furthermore, the two-dimensional peptide maps of the two molecules prepared from retinas cultured in the presence of [3H]glucosamine are identical. We conclude that gp90 is a turnover fragment of gp130 and comprises that portion of gp130 exposed at the cell surface. The relevance of these polypeptides to Ca2+-dependent retina cell-cell adhesion and their similarity to polypeptides implicated in Ca2+-dependent adhesion of other cell types are discussed.