Figure 6. There are five copies of Cag3 within the OMC asymmetric unit.

(a) Five different molecules of Cag3 are present within the asymmetric unit. The general architecture of Cag3 can be described as two domains, the proximal domain (residues 62–232) and the distal domain (residues 252–309). (b) All five of the Cag3 proteins share a heavily interwoven architecture in which β-sheets are formed between adjacent molecules within the asymmetric unit. (c) A topology diagram showing the general architecture of all copies of Cag3 within the asymmetric unit. (d) The interface of Cag3-2 (cyan) and Cag3-3 (light blue) is formed predominantly by two helices with contact mediated by Y114 and R128 of each molecule. (e) The interface of Cag3-3 (light blue) and Cag3-4 (blue) is dramatically different from the other Cag3 interfaces and includes an extensive hydrophobic interaction that is formed by two adjacent beta sheets. (f) Cag3-4 (blue) and Cag3-5 (navy blue) share a similar interface as Cag3-2 and Cag3-3, as shown above. (g) A view of the Cag T4SS is shown from the top-down (left) and indicates the position of a loop within the Cag3-1 proximal domain (amino acids 181–204, black box) that mediates contacts between asymmetric units. (h) The proximal domain of Cag3 contains a fold that is structurally similar to folds within CagT-1 (shown in red), CagT-2 (shown in salmon) and X. citri VirB7 (PDB 6GYB, shown in purple).

Figure 6—figure supplement 1. The repetitive β-sheet structure of CagT and Cag3.

(a) The table lists residues that contribute to six β-sheets formed by copies of CagT and Cag3 within the OMC. The location of these six β-sheets is indicated in panel (b). (c–e) Representative density for all β-sheets with landmark residues noted for each. (f) Representative density for β-sheet strand exchange is shown for β-sheet 3 (top, left), β-sheet 4 (top, right), β-sheet 5 (bottom, left), and β-sheet 6 (bottom, right).