Characterization of a nucleotide stimulated aspartic proteinase in rat liver plasma membranes

Inositol phosphoglycan molecules are believed to mediate multiple intracellular actions of insulin. They are released from plasma membranes in response to insulin binding and are transported into the cell. Release of insulin mediators is stimulated by MnATP and MgATP and is inhibited by p-aminobenzamidine. Inositol phosphoglycan mediators may be released by a poorly characterized mechanism requiring proteolytic cleavage of an attached protein from the mediator and phospholipase cleavage of the mediator from its membrane anchor. We examined rat liver plasma membranes for proteinase activity stimulated by insulin and MnATP. Although we could not demonstrate insulin stimulation, we have found and characterized a nucleotide-stimulated aspartic proteinase bound to rat liver plasma membranes. We also detected and separated a soluble activating factor for the proteinase. The activating factor appears to be a protein with M(r) approximately 70 kDa.