The Highly Repetitive Region of the Helicobacter pyloriCagY Protein Comprises Tandem Arrays of an α-Helical Repeat Module

摘要

The cag-pathogenicity-island-encoded type IV secretion system of Helicobacter pylori functions to translocate the effector protein CagA directly through the plasma membrane of gastric epithelial cells. Similar to other secretion systems, the Cag type IV secretion system elaborates a surface filament structure, which is unusually sheathed by the large cag-pathogenicity-island-encoded protein CagY. CagY is distinguished by unusual amino acid composition and extensive repetitive sequence organised into two defined repeat regions. The second and major repeat region (CagYrpt2) has a regular disposition of six repetitive motifs, which are subject to deletion and duplication, facilitating the generation of CagY size and phenotypic variants. In this study, we show CagYrpt2 to comprise two highly thermostable and acid-stable α-helical structural motifs, the most abundant of which (motif A) occurs in tandem arrays of one to six repeats terminally flanked by single copies of the second repeat (motif B). Isolated motifs demonstrate hetero- and homomeric interactions, suggesting a propensity for uniform assembly of discrete structural subunit motifs within the largerCagYrpt2 structure. Consistent with this, CagY proteinscomprising substantially different repeat 2 motif organisations demonstrateequivalent CagA translocation competence, illustrating a remarkable structuraland functional tolerance for precise deletion and duplication of motif subunits.We provide the first insight into the structural basis forCagYrpt2 assembly that accommodates both the variable motifsequence composition and the extensive contraction/expansion of repeat moduleswithin the CagYrpt2 region.