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Volume 272, Number 43, Issue of October 24, 1997 pp. 27464-27469
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Calcium-stimulated Phosphorylation of MAP-2 in Pancreatic TC3-cells Is Mediated by Ca²⁺/Calmodulin-dependent Kinase II

(Received for publication, March 11, 1997, and in revised form, August 12, 1997)

Kimberly A. Krueger , Harshika Bhatt , Michael Landt ^§ and Richard A. Easom

From the  Department of Biochemistry and Molecular Biology, University of North Texas Health Science Center, Fort Worth, Texas 76107-2699 and ^§ Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110

An understanding of the role of CaM kinase II in the pancreatic -cell is dependent on the identification of its cellular targets. One of the best substrates of CaM kinase II in vitro that could function in secretory events is the microtubule-associated protein, MAP-2. By immunoblot analysis, a high molecular weight protein with electrophoretic properties characteristic of MAP-2, was identified in rat insulinoma TC3 cells and isolated rat islets. In immunoprecipitation experiments employing -toxin-permeabilized TC3 cells, elevation of intracellular Ca²⁺ or addition of forskolin, an adenylate cyclase activator, induced significant phosphorylation of MAP-2 in situ. The effect of Ca²⁺ was rapid, concentration-dependent and closely correlated with activation of CaM kinase II under similar experimental conditions. H-89, a specific and potent inhibitor of cAMP-dependent protein kinase (PKA), prevented forskolin-induced MAP-2 phosphorylation but had little effect on MAP-2 phosphorylation stimulated by elevated Ca²⁺. Phosphopeptide mapping revealed that the phosphorylation pattern observed in situ upon incubation of the TC3 cells with increased free Ca²⁺, was strikingly similar to that generated in vitro by CaM kinase II, most notably with regard to the increased phosphate incorporated into one prominent site. These data provide evidence that MAP-2 is phosphorylated by CaM kinase II in the pancreatic -cell in situ, and that this event may provide an important link in the mediation of Ca²⁺-dependent insulin secretion.

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