Insulin stimulates the release of a subset of GPI-anchored proteins in a G-protein independent manner

The glycosyl-phosphatidylinositol anchored protein, membrane dipeptidase (E C 3.4.13.19) is released from the surface of 3T3-L1 adipocytes in response to insulin treatment through the action of a phospholipase C. The present s tudy investigates the role of guanine-nucleotide binding proteins (G-protei ns) in this process. Treatment of permeabilized 3T3-L1 adipocytes with GTP gamma S did not cause release of membrane dipeptidase into the medium, whil e GDP beta S did not inhibit the insulin-stimulated release of membrane dip eptidase. Other activators of G-proteins, including the tetradecapeptide ma stoparan, pertussis toxin and AIF(3), also caused no significant release of membrane dipeptidase from the surface of the 3T3-L1 adipocytes. From these observations it is concluded that G-proteins are not involved in the insul in-stimulated release of membrane dipeptidase. Although X-Pro aminopeptidas e (EC 3.4.11.9) is GPI-anchored in 3T3-L1 adipocytes as shown by digestion with bacterial phosphatidylinositol-specific phospholipase C, it was not re leased upon insulin treatment of the cells, indicating that only a subset o f the GPI-anchored proteins are susceptible to insulin-stimulated release.