Department of Biochemistry and Cellular and Molecular Biology, University of Tennessee, Knoxville, Tennessee, USA.
J Bacteriol. 2023 Jun 27;205(6):e0048422. doi: 10.1128/jb.00484-22. Epub 2023 May 31.
Chemotaxis in and depends on the presence of hexagonal polar arrays composed of membrane-bound chemoreceptors that interact with rings of baseplate signaling proteins. In the alphaproteobacterium Azospirillum brasilense, chemotaxis is controlled by two chemotaxis signaling systems (Che1 and Che4) that mix at the baseplates of two spatially distinct membrane-bound chemoreceptor arrays. The subcellular localization and organization of transmembrane chemoreceptors in chemotaxis signaling clusters have been well characterized but those of soluble chemoreceptors remain relatively underexplored. By combining mutagenesis, microscopy, and biochemical assays, we show that the cytoplasmic chemoreceptors AerC and Tlp4b function in chemotaxis and localize to and interact with membrane-bound chemoreceptors and chemotaxis signaling proteins from both polar arrays, indicating that soluble chemoreceptors are promiscuous. The interactions of AerC and Tlp4b with polar chemotaxis signaling clusters are not equivalent and suggest distinct functions. Tlp4b, but not AerC, modulates the abundance of chemoreceptors within the signaling clusters through an unknown mechanism. The AerC chemoreceptor, but not Tlp4b, is able to traffic in and out of chemotaxis signaling clusters depending on its level of expression. We also identify a role of the chemoreceptor composition of chemotaxis signaling clusters in regulating their polar subcellular organization. The organization of chemotaxis signaling proteins as large membrane-bound arrays underlies chemotaxis sensitivity. Our findings suggest that the composition of chemoreceptors may fine-tune chemotaxis signaling not only through their chemosensory specificity but also through their role in the organization of polar chemotaxis signaling clusters. Cytoplasmic chemoreceptors represent about 14% of all chemoreceptors encoded in bacterial and archaeal genomes, but little is known about how they interact with and function in large polar assemblies of membrane-bound chemotaxis signaling clusters. Here, we show that two soluble chemoreceptors with a role in chemotaxis are promiscuous and interact with two distinct membrane-bound chemotaxis signaling clusters that control all chemotaxis responses in Azospirillum brasilense. We also found that any change in the chemoreceptor composition of chemotaxis signaling clusters alters their polar organization, suggesting a dynamic interplay between the sensory specificity of chemotaxis signaling clusters and their polar membrane organization.
趋化作用依赖于由膜结合趋化受体组成的六边形极性阵列的存在，这些受体与底板信号蛋白的环相互作用。在α变形菌 Azospirillum brasilense 中，趋化作用受两个趋化信号系统（Che1 和 Che4）控制，这两个系统在两个空间上不同的膜结合趋化受体阵列的底板处混合。趋化信号簇中跨膜趋化受体的亚细胞定位和组织已得到很好的描述，但可溶性趋化受体的研究相对较少。通过结合诱变、显微镜和生化测定，我们表明细胞质趋化受体 AerC 和 Tlp4b 在趋化作用中起作用，并定位于两个极性趋化受体阵列的膜结合趋化受体和趋化信号蛋白，并相互作用，表明可溶性趋化受体是混杂的。AerC 和 Tlp4b 与极性趋化信号簇的相互作用并不等同，表明具有不同的功能。Tlp4b 但不是 AerC 通过未知机制调节信号簇中趋化受体的丰度。AerC 趋化受体但不是 Tlp4b 能够根据其表达水平在趋化信号簇内外运输。我们还确定了趋化信号簇中趋化受体的组成在调节其极性亚细胞组织中的作用。趋化信号蛋白作为大的膜结合阵列的组织是趋化敏感性的基础。我们的发现表明，趋化受体的组成不仅可以通过其化学感觉特异性，还可以通过它们在极性趋化信号簇组织中的作用来微调趋化信号。细胞质趋化受体约占细菌和古菌基因组中编码的所有趋化受体的 14%，但对于它们如何与大的极性膜结合趋化信号簇相互作用和发挥作用知之甚少。在这里，我们表明，在趋化作用中起作用的两种可溶性趋化受体是混杂的，并与控制 Azospirillum brasilense 中所有趋化作用反应的两个不同的膜结合趋化信号簇相互作用。我们还发现，趋化作用信号簇的趋化受体组成的任何变化都会改变它们的极性组织，这表明趋化作用信号簇的感官特异性与其极性膜组织之间存在动态相互作用。
