Fig. 5
From: Structural basis of the molecular ruler mechanism of a bacterial glycosyltransferase
Glycan counting mechanism during PglH reaction. PglH is shown schematically, with N-terminal and C-terminal domains shaded yellow and pink, respectively, and labeled. The ruler helix is shaded orange, and the three residues relevant to the counting mechanism are indicated. Phospholipids from leaflet of the lipid bilayer are depicted in gray. The polyprenyl tail of the LLO is depicted as a blue, curved line, phosphate moieties as a circled P, and the saccharide symbols are as in Fig. 1. State 0 corresponds to UDP-GalNAc-bound PglH as observed in our crystal structure. State 1 represents a ternary complex upon binding of Und-PP-Bac-GalNAc2 (tri-LLO), with the pyrophosphate group of tri-LLO interacting with the side chains of residues K75 and R72 as observed in the two crystal structures with LLO bound. Following transfer of the first GalNAc, the product Und-PP-Bac-GalNAc3 (Tetra-LLO) is predicted to move along the ruler helix, where the pyrophosphate moiety of the LLO will be coordinated mainly by the side chains of R72 (state 2). After the second transfer reaction, the product, Und-PP-Bac-GalNAc4 (Penta-LLO) is predicted to move along the ruler helix once more, where the pyrophosphate moiety of the LLO will be coordinated mainly by K68 (state 3). The shown interactions of the LLO with the ruler helix are inferred from functional experiments. Each transfer reaction involves an exchange of UDP for a UDP-GalNAc. Once the product Und-PP-Bac-GalNAc5 (Hexa-LLO) is released from PglH, it is processed by PglI, which adds a glucose moiety to the third GalNAc
