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 [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⁺). In the reaction, rhamnose is directly transferred to the side chain of a threonine (Fig. 2).

Figure 2. Catalysis mechanisms for RIN4 rhamnosylation by AvrB supported by crystal structures. (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 attack the rhamnose moiety in the co-substrate, UDP-rhamnose (Fig. 2). The threonine is close to a histidine and a threonine, which may stabilize the acceptor. UDP-rhamnose is stabilized by a few residues in the pocket.

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). AvrB shares similar structural features with other Fido proteins despite the primary sequences are divergent.

Family Firsts

First stereochemistry determination

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First catalytic nucleophile identification

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First general acid/base residue identification

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First 3-D structure

The first structure of GT138 family (Fido type) is the crystal structure of AvrB [3].

References

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