Glycosyltransferase Family 138

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• Author: Wei Peng
• Responsible Curator: Kim Orth

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Glycosyltransferase Family GT138
Clan	Fido fold
Mechanism	Inverting
Active site residues	Known
CAZy DB link
https://www.cazy.org/GT138.html

Substrate specificities

GT138 family of glycosyltransferase is exemplified by AvrB, a Fido protein (Fig. 1A) [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 modify the host protein RIN4 and causes the programmed cell death (namely hypersensitive response) [1, 2].

Kinetics and Mechanism

AvrB contains a Fido domain [3, 4] (Fig. 1A), different from other known glycosyltransferases containing folds of GT-A, GT-B, GT-C, lysozyme-type, GT101, and GT108 [5, 6, 7, 8] (Fig. 1B). Interestingly, Fido proteins can also be enzymes with activities of AMPylation [9], phosphorylation [10], UMPylation [11], and phosphocholination [12, 13]. Hence, AvrB is a unique Fido protein that functions as a glycosyltransferase.

Figure 1. Glycosyltransferase folds. (A) Fido fold (left [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.

The rhamnosylation reaction catalyzed by AvrB does not require divalent cations (e.g., Mg²⁺) [1]. In the reaction, rhamnose is directly transferred to the side chain of a threonine of RIN4, T166 (Fig. 2) [1].

Figure 2. Catalysis mechanisms for RIN4 rhamnosylation by AvrB supported by crystal structures [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. 2) [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. 2) [1].

Three-dimensional structures

AvrB represents the prototype for glycosyltransferases of Fido fold. AvrB contains a large internal domain between helix α2 and helix α3 (Fig. 1A) [1, 3, 4, 14]. AvrB shares similar structural features with other Fido proteins despite the primary sequences are divergent [4].

Family Firsts

The first member of GT138 family shown to be a glycosyltransferase is AvrB [1].

The first structure of GT138 family is AvrB [3]. A few AvrB structures are available to reveal the catalysis mechanisms [1, 3, 14]

References

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