Demonstration of a class of proteins loosely associated with secretory granule membranes

It is shown, in this study, that rat secretory granule membrane preparations, as prepared by the method of Amsterdam et al. [(1971) J. Cell Biol. 50, 187-200], contain a protein fraction which is removed by washing in isotonic medium. This fraction contains unusually high levels of Pro, Gly and Glx, and appears to label rapidly if the rats are pulsed with [14-c] amino acids prior to removal of the glands. The fraction, which may represent specifically adsorbed secretory protein(s) or peripheral membrane protein, is significant to investigators using this model system to study secretory phenomena.