Khandke K M, Fairwell T, Acharya A S, Manjula B N
Rockefeller University, New York, New York 10021.
J Protein Chem. 1990 Oct;9(5):511-22. doi: 10.1007/BF01025004.
Serologically distinct group A streptococcal M proteins, the antiphagocytic determinants of the bacteria, have a highly repetitive sequence and exhibit a heptad periodicity characteristic of alpha-helical coiled-coil proteins. Based on the differences in the pattern of hepatad periodicity, the coiled-coil region of the complete M molecule has been divided into three distinct domains: I, II, and III. Domains I and II together constitute the variable part of M protein, whereas domain III is conserved among serotypes. Pepsin treatment of the M5, M6, and M24 streptococci results in a preferential cleavage of their M molecules between the predicted domains II and III, releasing biologically active fragments of the respective M proteins. Thus, a pepsin cleavage site at the junction of their variable and conserved regions is conserved in the M5, M6, and M24 proteins. In contrast, in the case of the M49 streptococci, the primary site of pepsin cleavage was observed to be within the conserved region of the M49 molecule, rather than at the junction of its variable and conserved regions. Despite containing part of the conserved region, the PepM49 protein is significantly smaller than the pepsin fragments of the M5, M6, and M24 proteins, which contain only the variable regions. However, in addition to the major PepM49 species, the pepsin digest of the type-49 streptococci also contained a smaller fragment, PepM49/a, as a minor component. Its formation was extremely sensitive to the pH of pepsin digestion. PepM49/a, which retains both the propensity to attain an alpha-helical conformation and the opsonic antibody epitope of the M49 molecule, contains only domains I and II like the other PepM proteins. Thus, as in the M5, M6, and M24 proteins, a pepsin cleavage site at the junction of the variable and conserved regions is indeed present in the M49 molecule, but is much less accessible relative to the other serotypes. Thus, the pepsin cleavage sites in the M protein correlate quite well with the boundaries of structurally distinct domains reflected by the predictive analysis. These sites apparently represent the flexible/hinge regions of the molecule. PepM49/a is the least repetitive and the shortest of the M protein pepsin fragments isolated so far. These results suggest that the flexibility of the interdomain regions in M protein may be dependent on the molecular size of their variable domains.(ABSTRACT TRUNCATED AT 400 WORDS)
血清学上不同的A群链球菌M蛋白是该细菌的抗吞噬决定簇，具有高度重复序列，并呈现α-螺旋卷曲螺旋蛋白特有的七肽周期性。基于七肽周期性模式的差异，完整M分子的卷曲螺旋区域已被分为三个不同结构域：I、II和III。结构域I和II共同构成M蛋白的可变部分，而结构域III在各血清型中是保守的。用胃蛋白酶处理M5、M6和M24型链球菌会导致其M分子在预测的结构域II和III之间优先裂解，释放出相应M蛋白的生物活性片段。因此，在M5、M6和M24蛋白中，其可变区和保守区交界处的胃蛋白酶裂解位点是保守的。相比之下，对于M49型链球菌，观察到胃蛋白酶的主要裂解位点位于M49分子的保守区内，而不是在其可变区和保守区的交界处。尽管包含部分保守区，但PepM49蛋白明显小于M5、M6和M24蛋白的胃蛋白酶片段，后者仅包含可变区。然而，除了主要的PepM49种类外，49型链球菌的胃蛋白酶消化产物还含有一个较小的片段PepM49/a作为次要成分。其形成对胃蛋白酶消化的pH极为敏感。PepM49/a保留了形成α-螺旋构象的倾向以及M49分子的调理素抗体表位，与其他PepM蛋白一样仅包含结构域I和II。因此，与M5、M6和M24蛋白一样，M49分子中确实存在可变区和保守区交界处的胃蛋白酶裂解位点，但相对于其他血清型而言，其可及性要低得多。因此，M蛋白中的胃蛋白酶裂解位点与预测分析所反映的结构不同结构域的边界相当吻合。这些位点显然代表了分子的柔性/铰链区。PepM49/a是迄今分离出的M蛋白胃蛋白酶片段中重复度最低且最短的。这些结果表明，M蛋白结构域间区域的柔性可能取决于其可变结构域的分子大小。（摘要截短至400字）
