Distribution of calmodulin and calmodulin-binding proteins in bovine pituitary: association of myosin light chain kinase with pituitary secretory granule membranes

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Abstract
Calcium is necessary for secretion of pituitary hormones. Many of the biological effects of Ca2+ are mediated by the Ca2+-binding protein calmodulin (CaM), which interacts specifically with proteins regulated by the Ca2+-CaM complex. One of these proteins is myosin light chain kinase (MLCK), a Ca2+-calmodulin dependent enzyme that phosphorylates the regulatory light chains of myosin, and has been implicated in motile processes in both muscle and non-muscle tissues. We determined the content and distribution of CaM and CaM-binding proteins in bovine pituitary homogenates, and subcellular fractions including secretory granules and secretory granule membranes. CaM measured by radioimmunoassay was found in each fraction; although approximately one-half was in the cytosolic fraction, CaM was also associated with the plasma membrane and secretory granule fractions. CaM-binding proteins were identified by an 251-CaM gel overlay technique and quantitated by densitometric analysis of the autoradiograms. Pituitary homogenates contained nine major CaM-binding proteins of 146, 131, 90, 64, 58, 56, 52, 31 and 22 kilodaltons (kDa). Binding to all the bands was specific, Cat+-sensitive, and displaceable with excess unlabeled CaM. Severe heat treatment (100°C, 15 min), which results in a 75% reduction in phosphodiesterase activation by CaM, markedly decreased 251I-CaM binding to all protein bands. Secretory granule membranes showed enhancement for CaM-binding proteins with molecular weights of 184, 146, 131, 90, and 52000. A specific, affinity purified antibody to chicken gizzard MLCK bound to the 146 kDa band in homogenates, centrifugal subcellular fractions, and secretory granule membranes. No such binding was associated with the granule contents. The enrichment of MLCK and other CaM-binding proteins in pituitary secretory granule membranes suggests a possible role for CaM and/or CaM-binding proteins in granule membrane function and possibly exocytosis.