The accessibility of certain proteins on embryonic chick neural retina cells to iodination and tryptic removal is altered by calcium

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Abstract
We have used cell-surface-specific labelling techniques and two-dimensional gel electrophoresis to identify proteins on embryonic chick neural retina cells and to determine the effects of Ca2+ on their accessibility to labelling and tryptic removal. A number of proteins on these cells are, in the presence of Ca2+, relatively inaccessible to iodination and/or tryptic removal. Of these, a glycoprotein of M, approx. 130 × 103, with a pl of approx. 4·8, is the major cellsurface-iodinatable species that is retained during trypsinization in the presence of Ca2+. The removal of Ca2+ renders this glycoprotein much more accessible to both procedures. Its accessibility to these probes decreases on re-addition of Ca2+. The accessibility of its oligosaccharide moiety to galactose oxidase is, however, unaltered by the removal of Ca2+. These characteristics, together with immunological data presented elsewhere suggest that this glycoprotein may be a component of the Ca2+-dependent adhesive system that can be demonstrated on these cells.