Fykse E M, Takei K, Walch-Solimena C, Geppert M, Jahn R, De Camilli P, Südhof T C
Department of Molecular Genetics, University of Texas Southwestern Medical School, Dallas 75235.
J Neurosci. 1993 Nov;13(11):4997-5007. doi: 10.1523/JNEUROSCI.13-11-04997.1993.
Synaptophysins are abundant synaptic vesicle proteins present in two forms: synaptophysin, also referred to as synaptophysin I (abbreviated syp I), and synaptoporin, also referred to as synaptophysin II (abbreviated syp II). In the present study, the properties and localizations of syp I and syp II were investigated to shed light on their relative functions. Our results reveal that syp II, similar to syp I, is an abundant, N-glycosylated membrane protein that is part of a heteromultimeric complex in synaptic vesicle membranes. Cross-linking studies indicate that syp II is linked to a low-molecular-weight protein in this complex as has been observed before for syp I. Furthermore, after transfection into CHO cells, syp II, similar to syp I, is targeted to the receptor-mediated endocytosis pathway. Immunocytochemistry of rat brain sections reveals that syp II expression is highly heterogeneous, with high concentrations of syp II only in selected neuronal populations, whereas syp I is more homogeneously expressed in most nerve terminals. In general, nerve terminals expressing syp II also express syp I. In addition to high levels of syp II observed in selected neurons, a rostrocaudal gradient of syp II expression was observed in the cerebellar cortex. Immunoelectron microscopy confirmed that syp II is localized to synaptic vesicles. Immunoprecipitations of synaptic vesicles from rat brain with antibodies to syp I demonstrated that syp II is colocalized with syp I on the same vesicles. However, after detergent solubilization, no coimmunoprecipitations of the two proteins were observed, suggesting that they are not complexed with each other although they are on the same vesicles. Together our results demonstrate that syp I and syp II have similar properties and are present on the same synaptic vesicles but do not coassemble. The presence of the two proteins in the same nerve terminal suggests that they have similar but nonidentical functions and that the relative abundance of the two proteins may contribute to the functional heterogeneity of nerve terminals.
突触素是丰富的突触小泡蛋白，以两种形式存在：突触素，也称为突触素I（缩写为syp I），以及突触孔蛋白，也称为突触素II（缩写为syp II）。在本研究中，对syp I和syp II的特性及定位进行了研究，以阐明它们的相对功能。我们的结果表明，syp II与syp I相似，是一种丰富的、N - 糖基化膜蛋白，是突触小泡膜中异源多聚体复合物的一部分。交联研究表明，syp II在该复合物中与一种低分子量蛋白相连，这与之前观察到的syp I情况相同。此外，转染到CHO细胞后，syp II与syp I相似，靶向受体介导的内吞途径。大鼠脑切片的免疫细胞化学显示，syp II表达高度异质性，仅在选定的神经元群体中有高浓度的syp II，而syp I在大多数神经末梢中表达更为均匀。一般来说，表达syp II的神经末梢也表达syp I。除了在选定神经元中观察到高水平的syp II外，在小脑皮质中还观察到syp II表达的头尾梯度。免疫电子显微镜证实syp II定位于突触小泡。用抗syp I抗体对大鼠脑突触小泡进行免疫沉淀表明，syp II与syp I共定位于同一小泡上。然而，去污剂溶解后，未观察到两种蛋白的共免疫沉淀，这表明它们虽然在同一小泡上，但并不相互结合。我们的结果共同表明，syp I和syp II具有相似的特性，存在于同一突触小泡上，但不共同组装。两种蛋白在同一神经末梢中的存在表明它们具有相似但不相同的功能，并且两种蛋白的相对丰度可能导致神经末梢的功能异质性。
