Membrane targeting signals of the bacterial SRP system

大肠杆菌的信号识别颗粒由一个48kda的蛋白Ffh和一个4.5S的RNA组成。具有不可切割信号序列的膜蛋白被识别为底物，靶向于膜，并在与SRP受体蛋白FtsY接触后释放。我们最近发现，与Sec/ ydc无关的膜传感器蛋白KdpD被SRP靶向，而KdpD- gfp融合蛋白分别根据Ffh或FtsY的存在定位在膜上。此外，在缺乏Ffh或FtsY的菌株中，KdpD蛋白不会插入到膜中。在KdpD蛋白中，在残基22-48的氨基端胞质结构域发现了赋予SRP相互作用的信息。进一步的实验表明，该序列足以与SRP结合并将GFP引导到膜上。我们现在想分析信号与SRP的哪个区域结合，并确定相关的序列特异性。与SRP共结晶应该给出相互作用的分子细节。与其他膜蛋白的序列比较应该回答这些信号是否普遍存在于具有大氨基末端亲水区域的膜蛋白中。我们想要解决的另一个问题是如何决定srp靶向蛋白是直接指向Sec转位酶、YidC膜插入酶还是直接指向双分子层。一个有趣的可能性是，每个靶标的信号是不同的，并指定与SecYEG， YidC或直接与双层的相互作用。

The signal recognition particle of Escherichia coli consists of a 48-kDa protein Ffh and a 4.5S RNA. Membrane proteins that have uncleavable signal sequences are recognized as substrates, are targeted to the membrane and are released after contacting the SRP receptor protein FtsY. We recently found that the Sec/YidC-independent membrane sensor protein KdpD is targeted by SRP and a KdpD-GFP fusion protein localizes at the membrane depending on the presence of Ffh or FtsY, respectively. In addition, the KdpD protein is not inserted into the membrane in strains that are deficient for Ffh or FtsY. The information within the KdpD protein that confers SRP interaction was found in the amino-terminal cytoplasmic domain of KdpD in the residues 22-48. Further experiments suggest that this sequence is sufficient to bind to SRP and to direct GFP to the membrane. We now want to analyse to which region of SRP the signal binds and determine the relevant sequence specificity. Co-crystallization with SRP should give the molecular details of the interaction. Sequence comparisons with other membrane proteins should answer whether such signals are ubiquitous for membrane proteins that have large amino-terminal hydrophilic regions. Another aspect we want to address is the question how the decision for the SRP-targeted protein is made to be either directed to the Sec translocase, to the YidC membrane insertase or straight to the bilayer. An interesting possibility is that the signal for each of the targets is different and specifies the interaction with SecYEG, YidC or directly with the bilayer.