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The GT138 family of glycosyltransferases is exemplified by AvrB [1]. As a bacterial effector from the plant pathogen Pseudomonas syringae, AvrB utilizes host UDP-rhamnose (or dTDP-rhamnose in vitro) as a co-substrate to rhamnosylate the host protein RIN4, and causes the programmed cell death (i.e. the hypersensitive response) [1, 2]. AvrB contains a "Fido" domain [3, 4], different from other known glycosyltransferases (see below). Interestingly, Fido proteins can also have AMPylation [5], phosphorylation [6], UMPylation [7], and phosphocholination [8, 9] activities. Hence, AvrB is a unique Fido protein that functions as a glycosyltransferase.
Kinetics and Mechanism
In the reaction, rhamnose is directly transferred to the side chain of a threonine of RIN4, T166 (Fig. 1) [1]. The rhamnosylation reaction catalyzed by AvrB does not require divalent cations (e.g., Mg2+) [1].
Figure 1. Catalysis mechanisms for RIN4 rhamnosylation by AvrB supported by crystal structures (image from [1]). (A) AvrB bound with RIN4. (B) UDP-rhamnose bound with AvrB and RIN4. (C) Rhamnose transferred to T166 of RIN4. (D) Release of rhamnosylated RIN4.
Catalytic Residues
A threonine (T166) from the protein substrate directly attacks the rhamnose moiety in the co-substrate, UDP-rhamnose (Fig. 1) [1]. The threonine is close to a histidine and a threonine in AvrB, which may stabilize the acceptor. UDP-rhamnose is stabilized by a few residues in the pocket (Fig. 1) [1].
Three-dimensional structures
AvrB represents the prototype for glycosyltransferases comprised of a Fido fold (Fig. 2A) [1]. Most glycosyltransferases contain GT-A, GT-B, GT-C, lysozyme-type, GT101, or GT108 folds (Fig. 2B) [10, 11, 12, 13]. AvrB contains a large internal domain between helix α2 and helix α3 (Fig. 2A) [1, 3, 4, 14]. AvrB shares similar structural features with other Fido proteins despite substantial primary sequence divergence [4].
Figure 1. Glycosyltransferase folds. (A) Fido fold (left, image from [4]) is found in diverse enzymes including AvrB (right), which is a distinct glycosyltransferase. (B) Other known glycosyltransferases contain folds of GT-A, GT-B, GT-C, lysozyme-type, GT101, and GT108. PDB codes are provided for representative structures.
Family Firsts
The first member of GT138 family shown to be a glycosyltransferase is AvrB [1].
The first structure of GT138 family is AvrB (Fig. 2A) [3]. A few AvrB structures are available to reveal the catalysis mechanisms [1, 3, 14]
