Switching in the self-assembly of tobacco mosaic virus.
Switching in the self-assembly of tobacco mosaic virus.
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DOI：
10.1016/0065-227x(90)90011-h
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发表时间：
1990-01-01
影响因子：
--
通讯作者：
Namba, K
中科院分区：
文献类型：
--
作者：
Caspar, D L;Namba, K
对TMV蛋白组装体的结构和物理化学性质的实验观察导致了自组装过程模型的根本转变：而不是由假设的双层盘成核，病毒组装似乎是通过特定RNA序列与病毒中排列的外壳蛋白的短螺旋聚集体的相互作用而启动的。20 s成核聚集体的形成涉及每个蛋白质亚基平均结合半个质子。该质子结合位点可以通过在病毒螺旋中以58 A的半径在顶部和底部蛋白质表面之间形成的羧基-羧酸对来鉴定。因为20 s聚集体由大约两个螺旋圈组成，所以在每对上下蛋白质亚基之间仅形成一个羧基-羧酸对。在中性pH下的20 s螺旋聚集体的长度的限制可以通过蛋白质链的内环的无序来解释，这是由于在有序的病毒结构中在25 A半径处形成异常质子结合位点的羧基之间的静电排斥。为了生长超过两到三圈，螺旋内部的蛋白质内环必须以紧密排列的方式排列。在中性pH下，通过与病毒RNA结合，通过与钙结合，或通过与质子在微酸性溶液中结合，可以克服与这种有序相反的静电排斥。感染后的病毒解体似乎是由于与细胞外环境相比细胞内钙和质子浓度低，这增加了参与钙和质子结合的带负电荷基团之间的静电排斥，从而允许细胞核糖体竞争性结合病毒RNA。在碱性溶液中，在高离子强度下形成的TMV蛋白的盘状聚集体似乎不参与病毒组装。叠盘聚集体，这是以前被假定为建立一个极性堆叠的假设极性两层聚集体，是，事实上，一个双极结构。由于盘状结构的匝之间的键合不同于病毒螺旋的键合，因此不会发生通过假定的位错在这些结构之间直接转换。TMV装配似乎涉及保守的结合特异性，如最初推测的，但仅在螺旋包装安排的蛋白质亚基。从无序到有序的蛋白质构象的转换，依赖于蛋白质亚基之间的静电相互作用的变化，似乎是控制组装过程的关键。
Experimental observations on the structure and physicochemical properties of TMV protein assemblies have led to a fundamental switch in the model of the self-assembly process: rather than being nucleated by the hypothetical two-layer disk, virus assembly appears to be initiated by interaction of the specific RNA sequence with a short helical aggregate of the coat protein arranged as in the virus. Formation of the 20s nucleating aggregate involves the binding of an average of half a proton per protein subunit. This proton-binding site can be identified with the carboxyl-carboxylate pair that is formed between top and bottom protein surfaces at a radius of 58 A in the virus helix. Because the 20s aggregate consists of about two helical turns, only one carboxyl-carboxylate pair will be formed between each top-bottom pair of protein subunits. Limitation of the length of the 20s helical aggregate at neutral pH can be accounted for by disorder of the inner loop of the protein chain, due to electrostatic repulsion among the carboxyl groups that form the anomalous proton-binding site at 25 A radius in the ordered virus structure. To grow beyond two to three turns, inner loops of the protein at the interior of the helix must be ordered in the close-packed arrangement. The electrostatic repulsion opposing this ordering can be overcome by binding of the viral RNA at neutral pH, by calcium binding, or by proton binding in slightly acid solution. Virus disassembly upon infection appears to result from the low intracellular calcium and proton concentration compared to the extracellular environment, which increases the electrostatic repulsion among the negatively charged groups involved in calcium and proton binding, thereby allowing cellular ribosomes to competitively bind the viral RNA. Disk aggregates of TMV protein, which form at high ionic strength in alkaline solution, do not appear to be involved in virus assembly. The stacked-disc aggregate, which was previously presumed to be built of a polar stack of the hypothetical polar two-layer aggregate, is, in fact, a bipolar structure. Because the bonding between turns of the disc structures is different from that of the virus helix, direct switching between these structures by the postulated dislocation does not occur. TMV assembly does appear to involve conservation of bonding specificity, as initially presumed, but only in helical packing arrangements of the protein subunits. Switching from disordered to ordered conformations of the protein, dependent on changes in the electrostatic interactions among the protein subunits, appears to be critical in controlling the assembly process.