Nikaido H
Department of Molecular and Cell Biology, University of California, Berkeley 94720.
Mol Microbiol. 1992 Feb;6(4):435-42. doi: 10.1111/j.1365-2958.1992.tb01487.x.
Porins and specific channels both produce water-filled pores that allow the transmembrane diffusion of small solutes, but the latter contain specific ligand-binding sites within the channels. Recent structural studies show that many or most of these proteins exist as beta-barrels with the beta-strands traversing the thickness of the outer membrane. The channels often have diameters in the range of 1 nm, and thus the penetration rates of solutes through porin channels are likely to be affected strongly by what appear to be minor differences in the size, shape, hydrophobicity or charge of the solute molecule. With the specific channels, the presence of binding sites can accelerate very significantly the diffusion of some ligands when they are present at low concentrations. Thus these simple channels can sometimes achieve a surprising degree of real or apparent specificity. Recent data tend to favour the idea that these proteins are first exported into the periplasm, and then inserted into the outer membrane. Although lipopolysaccharides seem to play a significant role in the final assembly of the trimeric porins, the details of the targeting process still remain to be elucidated.
孔蛋白和特异性通道都能产生充满水的孔，允许小溶质进行跨膜扩散，但后者在通道内含有特异性配体结合位点。最近的结构研究表明，这些蛋白质中的许多或大多数以β桶的形式存在，β链贯穿外膜的厚度。通道的直径通常在1纳米范围内，因此溶质通过孔蛋白通道的渗透速率很可能受到溶质分子在大小、形状、疏水性或电荷上看似微小差异的强烈影响。对于特异性通道，当某些配体浓度较低时，结合位点的存在能显著加速它们的扩散。因此，这些简单的通道有时能实现令人惊讶程度的实际或表观特异性。最近的数据倾向于支持这样一种观点，即这些蛋白质首先被输出到周质中，然后插入到外膜中。尽管脂多糖似乎在三聚体孔蛋白的最终组装中起重要作用，但靶向过程的细节仍有待阐明。
