Steiner J P, Ling E, Bennett V
J Biol Chem. 1987 Jan 15;262(2):905-14.
Synapsin I, a major neuron-specific substrate for cAMP-dependent and Ca2+/calmodulin-dependent protein kinases, associates in in vitro assays with brain integral membrane protein site(s) distinct from secretory vesicles and with the neurofilament Mr = 68,000 subunit. The membrane sites for synapsin involve protein(s) and are likely to have physiological relevance since the binding of 125I-labeled synapsin is abolished by digestion with chymotrypsin, is displaced by unlabeled synapsin, is of high affinity (KD = 10 nM), and has a capacity (42 pmol/mg membrane protein) that is comparable to the amount of synapsin in brain, optimal binding occurs at physiological pH (6.8-7.2) and salt concentrations (50 mM), and synapsin binding to membranes is inhibited by phosphorylation with Ca2+/calmodulin-dependent protein kinase. The brain membrane protein sites for synapsin are not due to synaptic vesicles, since synaptic vesicles do not sediment under the conditions of the binding assay. Association between synapsin and the Mr = 68,000 neurofilament subunit has also been demonstrated. The binding of synapsin with the neurofilament subunit is specific since this binding interaction is saturable, with a 1:1 stoichiometry, the binding involves only certain proteolytically derived domains of synapsin, and is therefore not a simple electrostatic interaction between the basic domains of synapsin and the acidic regions in the neurofilament subunit, and Ca2+/calmodulin-dependent phosphorylation of synapsin inhibits this interaction. Synapsin promotes cross-linking of synaptic vesicles to brain membranes, and these complexes are reduced by phosphorylation of synapsin. This interconnecting function of synapsin may be a general characteristic of synapsin binding, with a membrane (synaptic vesicle or nonsecretory vesicle)-bound synapsin associating with microtubules, neurofilaments, or spectrin.
突触素I是一种主要的神经元特异性底物，可被cAMP依赖性和Ca2+/钙调蛋白依赖性蛋白激酶作用，在体外试验中，它与不同于分泌囊泡的脑整合膜蛋白位点以及分子量为68,000的神经丝亚基相结合。突触素的膜位点涉及蛋白质，并且可能具有生理相关性，因为用胰凝乳蛋白酶消化可消除125I标记的突触素的结合，未标记的突触素可使其移位，具有高亲和力（KD = 10 nM），并且具有与脑中突触素量相当的容量（42 pmol/mg膜蛋白），最佳结合发生在生理pH（6.8 - 7.2）和盐浓度（50 mM）下，并且Ca2+/钙调蛋白依赖性蛋白激酶的磷酸化可抑制突触素与膜的结合。突触素的脑膜蛋白位点不是由于突触囊泡，因为在结合试验条件下突触囊泡不会沉淀。突触素与分子量为68,000的神经丝亚基之间的结合也已得到证实。突触素与神经丝亚基的结合是特异性的，因为这种结合相互作用是可饱和的，化学计量比为1:1，该结合仅涉及突触素的某些经蛋白水解衍生的结构域，因此不是突触素的碱性结构域与神经丝亚基中的酸性区域之间的简单静电相互作用，并且突触素的Ca2+/钙调蛋白依赖性磷酸化会抑制这种相互作用。突触素促进突触囊泡与脑膜的交联，并且这些复合物会因突触素的磷酸化而减少。突触素的这种相互连接功能可能是突触素结合的一般特征，膜结合的（突触囊泡或非分泌囊泡）突触素与微管、神经丝或血影蛋白相关联。
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