Insulin-dependent protein phosphorylation in membranes. Isolation and characterization of a phosphorylated proteolipid from sarcolemma

Phosphorylation of multiple sites in a 15,000 dalton proteolipid from rat skeletal muscle sarcolemma, catalyzed by adenosine 3',5'-monophosphate-dependent and calcium/phospholipid-dependent protein kinases

This study reports a partial characterization of a 15,000 dalton (15 kDa) proteolipid present in rat skeletal muscle sarcolemma. The proteolipid is phosphorylated by both cyclic AMP-dependent and calcium/phospholipid-dependent protein kinases, displays an isoelectric point (pI) of 5.9, and can be extracted from sarcolemma by acidified chloroform/methanol (2:1) or non-ionic detergents. Phosphoamino acid analysis and tryptic fingerprinting of the phosphorylated proteolipid indicate that both cyclic AMP- and calcium/phospholipid-dependent protein kinases predominantly phosphorylate serine residue(s) on a single tryptic peptide. Additivity experiments and thermolytic fingerprinting demonstrate a minimum of two distinct phosphorylation sites on the proteolipid, the phosphorylation of which is independently catalyzed by cyclic AMP-dependent and calcium/phospholipid-dependent protein kinases in vitro. This sarcolemma proteolipid, which appears to be identified to a sarcolemma protein previously reported to be phosphorylated upon addition of insulin in a GTP-dependent manner (Walaas, O., Walaas, E., Rye-Alertsen, A. and Horn, R.S. (1979) Mol. Cell. Endocrinol. 16, 45-55), therefore represents a possible membrane target for those neuronal and hormonal stimuli which can regulate cyclic AMP-dependent or calcium/phospholipid-dependent protein kinase activities in skeletal muscle.

Evidence that insulin and guanosine triphosphate regulate dephosphorylation of the beta-subunit of the insulin receptor in sarcolemma membranes isolated from skeletal muscle

When sarcolemma membranes isolated from rat skeletal muscle were incubated with [gamma-32P]ATP, a membrane protein of apparent Mr 95,000 was rapidly phosphorylated, with the 32P content reaching a maximum within 2 s. On the basis of immunoprecipitation with anti-insulin-receptor antiserum, phosphoamino acid analysis and Mr, this protein probably represents the beta-subunit of the insulin receptor. Similarly, on incubation of the membrane with adenosine 5'-[gamma-[35S]thio] triphosphate the 95 kDa protein was thiophosphorylated, indicating thiophosphorylation of the beta-subunit of the insulin receptor on the basis of immunoprecipitation studies. The effect of insulin on the phosphorylation of this protein in the membrane was studied. Insulin induced a 20% decrease in the 32P labelling of the protein when the membranes were phosphorylated for 10 s. This insulin effect was dose-dependent, with half-maximal effect obtained at 2-3 nM-insulin. Addition of GTP, but not GDP or guanosine 5'-[beta, gamma-imido]triphosphate, enhanced the effect to 35% inhibition, with half-maximal effect of GTP obtained at 0.5 microM. GTP had no effect on the phosphorylation of the protein in the absence of insulin. Analysis of this insulin effect showed that insulin increased the rate of dephosphorylation of the 95 kDa protein in the membrane. In contrast, insulin had no effect on thiophosphorylation of the 95 kDa membrane protein after incubation with adenosine 5'-[gamma-[35S]thio]triphosphate. Since thiophosphorylated proteins are less sensitive to phosphatase action, these investigations suggest that insulin stimulated a protein phosphatase activity in a GTP-dependent manner. The possibility that GTP-regulatory proteins are involved in the action of insulin on the phosphorylation of the insulin receptor and other membrane proteins is discussed.

Insulin secretion and action

Recent advances in insulin secretion indicate that pertussis toxin abolishes the inhibition by alpha 2 adrenoceptor activation of insulin release by the pancreas. Pertussis toxin adenosine diphosphate (ADP) ribosylates an inhibitory guanine nucleotide-binding protein (Ni) involved in inhibition of adenylate cyclase. The decrease in cyclic adenosine monophosphate (AMP) by epinephrine may account for its inhibition of insulin release. Insulin interaction with its receptor results in an increase in the tyrosine protein kinase activity of the receptor. Second messengers for insulin are generated, hexose transport is accelerated, and a cyclic AMP-independent protein kinase is activated that phosphorylates at serinethreonine residues. The activity of membrane-bound enzymes such as adenylate cyclase and Ca2+-Mg2+-ATPase is affected. The relative importance of these effects of insulin in its regulation of cellular metabolism remains to be established.