Baitinger C, Willard M
Department of Anatomy and Neurobiology, Washington University School of Medicine, Saint Louis, Missouri 63110.
J Neurosci. 1987 Nov;7(11):3723-35. doi: 10.1523/JNEUROSCI.07-11-03723.1987.
Synapsin I is a neuronal phosphoprotein that is associated with the cytoplasmic surface of small, clear synaptic vesicles in neuronal synaptic terminals; it may play an important role in synaptic transmission. In vitro, it can interact with fodrin, a relative of the erythrocyte protein spectrin. We have investigated the delivery of synapsin I from its site of synthesis in neuronal cell bodies to synaptic terminals by means of the process of axonal transport. We labeled the newly synthesized proteins of rabbit retinal ganglion cells by injecting 35S-methionine into the vitreous humour, and subsequently observed the appearance of radioactive synapsin I (identified by its 2-dimensional electrophoretic mobility) in tissues containing the axons and synaptic terminals of these neurons. A portion of the newly synthesized synapsin I was axonally transported at the velocity of the most rapidly transported (group I) proteins, which comprise membrane-associated proteins and may include elements of synaptic vesicles. However, the subsequent time course of labeling of synapsin I in the axons suggests that greater than 90% of the axonally transported synapsin I may comprise 2 additional populations--one transported rapidly, the other slowly--that are released from the cell bodies only after a delay of more than 1 d. The delayed, slowly transported population moves at the velocity (approximately 6 mm/d) of groups III and IV (which include fodrin and other proteins of the membrane cytoskeleton). We consider whether such distinct populations may correspond to functionally specialized variants of synapsin I-like proteins that may be transported in association with different organelles. The electrophoretic mobility of labeled synapsin I-like proteins in the axons changed subtly with time. Additional subtle differences between labeled synapsin I-like proteins in the axons and the terminal-containing tissues suggest that certain posttranslational modifications occur specifically in the terminals.
突触素I是一种神经元磷蛋白，与神经元突触终末中小而清亮的突触小泡的胞质面相关联；它可能在突触传递中起重要作用。在体外，它能与血影蛋白（一种红细胞蛋白血影素的相关蛋白）相互作用。我们通过轴突运输过程研究了突触素I从其在神经元细胞体中的合成部位向突触终末的运输。我们通过向玻璃体内注射35S-甲硫氨酸来标记兔视网膜神经节细胞新合成的蛋白质，随后在含有这些神经元轴突和突触终末的组织中观察到放射性突触素I（通过其二维电泳迁移率鉴定）的出现。新合成的突触素I的一部分以运输速度最快的（I组）蛋白质的速度进行轴突运输，这些蛋白质包括膜相关蛋白，可能还包括突触小泡的成分。然而，轴突中突触素I标记的后续时间进程表明，超过90%的经轴突运输的突触素I可能包含另外两个群体——一个快速运输，另一个缓慢运输——它们仅在延迟超过1天后才从细胞体释放。延迟的、缓慢运输的群体以III组和IV组（包括血影蛋白和膜细胞骨架的其他蛋白质）的速度（约6毫米/天）移动。我们考虑这些不同的群体是否可能对应于与不同细胞器相关运输的突触素I样蛋白的功能特化变体。轴突中标记的突触素I样蛋白的电泳迁移率随时间发生细微变化。轴突中标记的突触素I样蛋白与含终末组织之间的其他细微差异表明，某些翻译后修饰专门发生在终末。
