Evidence for the existence of actomyosin ATPase in the rat pancreas. Isolation and biochemical characterization

Purification and characterization of myosin light-chain kinase from the rat pancreas

We have partially purified a protein kinase from rat pancreas that phosphorylates two light-chain subunits of pancreatic myosin, a doublet with components of 18 and 20 kDa. This protein kinase was purified approx. 1000-fold by sequential (NH4)2SO4 fractionation, gel filtration, ion-exchange and affinity chromatography on calmodulin-Sepharose 4B. The resultant enzyme preparation is free of cyclic AMP-dependent protein kinase, protein kinase C and calmodulin-dependent type I or II kinase activities. The purified protein kinase is completely dependent on Ca2+ and calmodulin, and phosphorylates a 20 kDa light-chain subunit of intact gizzard myosin, suggesting that it belongs to a class of enzymes known as myosin light-chain kinase (MLCK). The apparent Km values of the putative pancreatic MLCK for ATP (73 microM), gizzard myosin light chains (18 microM) and calmodulin (2 nM) are similar to those reported for MLCKs isolated from smooth muscle, platelet and other sources. The enzyme is half-maximally activated at a free Ca2+ concentration of 2.5 microM. A single component of the affinity-purified kinase reacts with antibodies to turkey gizzard MLCK. The apparent molecular mass of this component is 138 kDa. Immunoprecipitation of a pancreatic homogenate with these antibodies decreases calmodulin-dependent kinase activity for pancreatic myosin by over 85%. The immunoprecipitate contains a single electrophoretic band of 138 kDa. Tryptic phosphopeptide analyses of pancreatic myosin, phosphorylated by either gizzard or pancreatic MLCK, are identical. Thus the enzyme that we have purified from rat pancreas is a MLCK, as judged by (1) absolute dependence on Ca2+ and calmodulin, (2) high affinity for calmodulin, (3) narrow substrate specificity for the light-chain subunit of myosin, and (4) reactivity with antibodies to turkey gizzard MLCK. These studies establish the existence of a pancreatic MLCK which may be responsible for regulating myosin phosphorylation and enzyme secretion in situ.

Myosin from pancreatic acinar carcinoma cells. Isolation, characterization and demonstration of heavy- and light-chain phosphorylation

Myosin has been identified in a variety of non-muscle cells, and is believed to play a role in maintenance of cell shape, locomotion, cytokinesis, exocytosis and other cellular functions. In this paper we describe the purification of myosin from a pancreatic acinar-cell carcinoma of the rat which forms solid tumours, but retains many differentiated functions. The purified myosin was composed of a 200,000 Da heavy chain and two or three classes of light chains. Electron-microscopic examination of rotary-shadowed preparations revealed that individual molecules had two globular heads and a long tail measuring approx. 149 nm. The myosin was soluble in high-salt buffers and became sedimentable as the ionic strength was lowered. Examination of negative-stained preparations showed that this sedimentable myosin consisted of short, bipolar, thick filaments which had a strong tendency to aggregate in a head-to-head manner. The ATPase activity of the purified myosin was stimulated by EDTA or Ca2+, but not by Mg2+. In low ionic strength the Mg2+-dependent ATPase activity was activated by muscle f-actin. The pancreatic myosin bound to actin and could be dissociated by the addition of MgATP. Myosin purified from cells cultured in media containing [32P]Pi was phosphorylated on one of the light chains as well as the heavy chain. Thus pancreatic acinar cells contain a typical non-muscle myosin, and the subunits of this molecule are subject to post-translational modification by phosphorylation.

Effector mechanisms of peptides of the VIP family

The present review is focused on the exocrine pancreas and liver where the only known effector mechanism of VIP is the activation of adenylate cyclase in plasma membranes. A two-state model of activation-deactivation of the enzyme visualizes the participation of VIP receptors and Ns, the guanyl nucleotide stimulatory protein of adenylate cyclase. In the rat pancreas, VIP and GRF receptors are indistinguishable and disulfide bridges influence their functional integrity. The antagonism of VIP and somatostatin perhaps requires, at the adenylate cyclase level, the contribution of Ni, the guanyl nucleotide inhibitory protein. The potentiation of VIP by various stimulants acting on Ca2+ movements may rely on later events, e.g., on a concerted activation of protein kinases. When comparing quantitatively peptide binding to receptors with adenylate cyclase activation, cyclic AMP levels and amylase secretion, a tool is at hand to tailor synthetic agonists and antagonists of VIP, with appropriate changes in the N-terminal moiety of the peptide (a good agonist allows efficient coupling of receptors to the adenylate cyclase system). Apart from stimulus-secretion coupling, VIP may influence protein synthesis in the rat pancreas, through the phosphorylation of ribosomal protein S6, and may alter the activity of the endoplasmic reticulum via the phosphorylation of Mr = 21 kDa and Mr = 25 kDa proteins. In rat liver membranes, high affinity VIP receptors are specifically labelled with 125I-helodermin and are coupled to adenylate cyclase (at variance with low affinity VIP receptors). These receptors are highly responsive to divalent cations and to guanyl nucleotides.

Phosphorylation of 3 particulate proteins in rat pancreatic acini in response to vasoactive intestinal peptide (VIP), secretin and cholecystokinin (CCK-8)

Rat pancreatic acini were preincubated with 0.4 mM 32Pi for 45 min at 37 degrees C, then exposed for 15 min to VIP, secretin or CCK-8. The incubation was terminated with a stop solution and a fraction rich in mitochondria and zymogen granules was separated from a microsome-rich fraction by differential centrifugation. After heating in the presence of SDS, beta-mercaptoethanol was added and the pattern of equivalent amounts of 32P-labelled proteins was examined by autoradiography of SDS-PAGE gels. VIP, secretin, and CCK-8 stimulated the phosphorylation of a Mr=33 K microsomal protein and that of two proteins of Mr=21 K and Mr=25 K mostly present in a fraction rich in mitochondria and zymogen granules. Stimulations were dose-dependent, the highest stimulant concentrations tested allowing 2- to 3-fold increases of phosphorylation over basal. When 1 nM CCK-8 was used simultaneously with 1 microM VIP, the cyclic AMP levels attained and the pattern of protein phosphorylation were similar to those obtained with VIP alone, and there was a potentiation of amylase secretion; when a supra-maximal 0.1 microM CCK-8 concentration was added, the VIP-induced elevation in cyclic AMP levels and the phosphorylation of the Mr=21 K and Mr=25 K proteins were partially antagonized, and no potentiation any more of secretion occurred. To conclude the in vitro phosphorylation of three particulate proteins (Mr=33 K, 25 K, and 21 K) was similarly increased in rat pancreatic acini in response to secretin and VIP (acting through cyclic AMP) and to CCK-8 (acting mostly through Ca2+).