Abstract：
Antigenically sequence variable M proteins of the major bacterial pathogen Streptococcus pyogenes (Strep A) are responsible for recruiting human C4b-binding protein (C4BP) to the bacterial surface, which enables Strep A to evade destruction by the immune system. The most sequence divergent portion of M proteins, the hypervariable region (HVR), is responsible for binding C4BP. Structural evidence points to the conservation of two C4BP-binding sequence patterns (M2 and M22) in the HVR of numerous M proteins, with this conservation applicable to vaccine immunogen design. These two patterns, however, only partially explain C4BP-binding by Strep A. Here, we identified several M proteins that lack these patterns but still bind C4BP, and determined the structures of two, M68 and M87 HVRs, in complex with a C4BP fragment. Mutagenesis of these M proteins led to identification of amino acids that are crucial for C4BP-binding, enabling formulation of new C4BP-binding patterns. Mutagenesis was also carried out on M2 and M22 proteins to refine or generate experimentally grounded C4BP-binding patterns. The M22 pattern was the most prevalent among M proteins, followed by the M87 and M2 patterns, while the M68 pattern was rare. These patterns, except for M68, were also evident in numerous M-like Enn proteins. Binding of C4BP via these patterns to Enn proteins was verified. We conclude that C4BP-binding patterns occur frequently in Strep A strains of differing M types, being present in their M or Enn proteins, or frequently both, providing further impetus for their use as vaccine immunogens.
摘要：
主要细菌病原体化脓性链球菌（StrepA）的抗原序列可变M蛋白负责将人C4b结合蛋白（C4BP）招募到细菌表面，这使得StrepA能够逃避免疫系统的破坏。M蛋白序列差异最大的部分，高变区（HVR），负责结合C4BP。结构证据表明，许多M蛋白的HVR中两种C4BP结合序列模式（M2和M22）的保守性，这种保守适用于疫苗免疫原设计。这两种模式，然而,仅部分解释了StrepA对C4BP的结合，我们鉴定了几种缺乏这些模式但仍与C4BP结合的M蛋白，并确定了两个的结构，M68和M87HVRs,与C4BP片段复合。这些M蛋白的诱变导致对C4BP结合至关重要的氨基酸的鉴定，能够制定新的C4BP结合模式。还对M2和M22蛋白进行诱变,以改进或产生实验基础的C4BP结合模式。M22模式是M蛋白中最普遍的，其次是M87和M2模式，而M68模式很少见。这些模式，除M68外，在许多M样Enn蛋白中也很明显。验证了C4BP通过这些模式与Enn蛋白的结合。我们得出结论，C4BP结合模式经常发生在不同M型的StrepA菌株中，存在于它们的M或Enn蛋白中，或者经常两者兼而有之，为它们作为疫苗免疫原的使用提供了进一步的动力。
