Fig. 9 | Nature Communications

Fig. 9

From: Crystal structure of undecaprenyl-pyrophosphate phosphatase and its role in peptidoglycan biosynthesis

Fig. 9

N-repeat and C-repeat in BacA viewed in isolation from within the membrane. a N-repeat. b C-repeat. Repeats include sequentially a re-entrant helix pair (the inner arch), a buttress helix and an outer arch. Repeats are shown with the same orientation they assume in the intact BacA. c Topology of BacA in simplified cylinder representation to illustrate how the N- and C-repeats in a and b interdigitate in the intact protein. It is possible that early in evolutionary time the individual N-repeat and C-repeat, in isolation, had functionality and stability. One would wonder if such isolated repeats could possibly be stable in a membrane with TM helices so far apart (to accommodate interdigitation in the intact, fused BacA protein). On inspection, transmembrane helices H3-H5 and H8-H10 are extremely hydrophobic along their full length suggesting that they could indeed be stable, as disposed in BacA, as isolated repeats or domains. But if isolated, they could presumably fold together reversibly in the membrane to form a helical bundle. Another question arises immediately, What happens to the re-entrant helices under these conditions? They too are extremely hydrophobic. However, the re-entrant loops L1–2 and L6–7, whilst not having many polar residues (FLPVSS in L1–2, WPGFS in L6–7), have peptide linkages in need of hydrogen bonding partners and thus are unlikely to be stable in isolation buried in the membrane’s apolar interior. One solution would be for each of the re-entrant helices to form a long transmembrane helix

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