Calmodulin binding proteins of the cholinergic electromotor synapse: synaptosomes, synaptic vesicles, receptor-enriched membranes, and cytoskeleton

Phosphorylation and cytoskeletal anchoring of the acetylcholine receptor by Src class protein-tyrosine kinases. Activation by rapsyn

Src class protein-tyrosine kinases bind to and phosphorylate the nicotinic acetylcholine receptor of skeletal muscle. This study provided evidence for the functional importance of Src kinases in regulating the nicotinic acetylcholine receptor at the neuromuscular junction. Three Src class kinases, Fyn, Fyk, and Src, each formed a complex with the endplate-specific cytoskeletal protein rapsyn. In addition, cellular phosphorylation by each kinase was stimulated by rapsyn in heterologous transfected cells. Several lines of evidence supported rapsyn as a substrate for Src kinases. Most importantly, rapsyn regulation of Fyn, Fyk, and Src resulted in phosphorylation of the nicotinic acetylcholine receptor beta and delta subunits and anchoring of the receptor to the cytoskeleton. Both nicotinic acetylcholine receptor phosphorylation and cytoskeletal anchoring were blocked by the Src kinase-selective inhibitor herbimycin A. Rapsyn alone also induced a modest increase in nicotinic acetylcholine receptor phosphorylation and cytoskeletal translocation. However, inhibition by herbimycin A and a catalytically inactive dominant negative Src demonstrated that the effects of rapsyn were mediated by endogenous Src kinases. These data support the importance of Src class kinases for stabilization of the nicotinic acetylcholine receptor at the endplate during synaptic differentiation at the neuromuscular junction.

The synaptic vesicle and the cytoskeleton

Images

Changes in the subcellular distribution of calmodulin-kinase II during brain development

Subcellular fractions prepared from rodent forebrain at different postnatal ages were examined for calmodulin-binding proteins using [125I]calmodulin and a gel overlay technique. Synaptic junction (SJ) fractions from newborn brain, which display purity comparable to adult SJ fractions, contain low but detectable amounts of 60 and 50 kdalton calmodulin-binding polypeptides; the latter being the major postsynaptic density protein. These polypeptides have recently been shown to be the calmodulin-binding protein subunits of calmodulin-dependent protein kinase II (CaM-kinase II). CaM-kinase II polypeptides represented the predominent calmodulin-binding proteins in nearly every subcellular fraction examined, regardless of postnatal age. Large increases were observed in the CaM-kinase II content of every subcellular fraction throughout postnatal development. During development, a striking shift in the subcellular distribution of CaM-kinase Ii was observed. Over 4 times as much CaM-kinase II was cytosolic relative to particulate in newborn brain while this ratio was completely reversed in adult brain. Large age-dependent increases in particulate-associated CaM-kinase II were observed in highly purified synaptic plasma membrane (5-fold) and SJ (14-fold) fractions. The CaM-kinase II content of SJ fractions increased approximately 70% between days 24 and 90, a period in development that follows the most active stages of synapse formation in situ. In adult brain, approximately 60% of CaM-kinase II in crude synaptosomal fractions (P2-INT) was recovered in SJ fractions. The CaM-kinase II in SPM fractions from all developmental ages resists solubilization in Triton X-100 and greater than 90% is recovered in SJ fractions. These studies indicate that during brain development the accumulation of SJ-associated CaM-kinase II represents an important process in the molecular and enzymatic maturation of CNS postsynaptic structures.

Characterization of a Mg2+-ATPase and a proton pump in cholinergic synaptic vesicles from the electric organ of Torpedo marmorata

Cholinergic synaptic vesicles from the electric organ of Torpedo marmorata are associated with a Mg2+-ATPase insensitive to ouabain and oligomycin. Treatment of vesicle membranes with dichloromethane releases a Mg2+-ATPase with apparent molecular mass of around 250 kDa as determined by gel filtration. The vesicular ATPase resembles the mitochondrial F1-ATPase in these properties. Gel electrophoresis of the solubilized ATPase shows however that only a single 50-kDa band is present as compared to the alpha-subunit (52 kDa) and beta-subunit (50 kDa) of electric organ mitochondrial F1-ATPase present in this range of molecular mass range. In agreement, covalent photoaffinity labelling of isolated vesicles with azido-ATP shows a 50-kDa band. Vesicle ghosts were found to accumulate [14C]methylamine in an ATP-dependent manner indicating the presence of an inwardly directed proton pump. We conclude that cholinergic vesicles contain a proton pump probably driven by the Mg2+-ATPase here described, which generates an electrochemical gradient across the vesicle membrane and is necessary for uptake and storage of acetylcholine within the vesicles.