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

Family features

GT138 family of glycosyltransferase is exemplified by AvrB [1]. AvrB contains a Fido domain (Fig. 1A) [2, 3], different from other known glycosyltransferases containing folds of GT-A, GT-B, GT-C, lysozyme-type, GT101, and GT108 (Fig. 1B) [4, 5, 6, 7].

Interestingly, Fido proteins can also be enzymes with activities of AMPylation [8], phosphorylation [9], UMPylation [10], and phosphocholination [11, 12]. Hence, AvrB is a unique Fido protein that functions as a glycosyltransferase.

Figure 1. Glycosyltransferase folds. (A) Fido fold (left, image from [3]) 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.

Substrate specificities

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 (namely hypersensitive response) [1, 13].

Kinetics and Mechanism

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

Figure 2. 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. 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 (Fig. 1A) [1]. AvrB contains a large internal domain between helix α2 and helix α3 (Fig. 1A) [1, 2, 3, 14]. AvrB shares similar structural features with other Fido proteins despite the primary sequences are divergent [3].

Family members

AvrB is the only well-studied member so far in the GT138 family [1].

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. 1A) [2]. A few AvrB structures are available to reveal the catalysis mechanisms [1, 2, 14]

References

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5. Lairson LL, Henrissat B, Davies GJ, and Withers SG. (2008). Glycosyltransferases: structures, functions, and mechanisms. Annu Rev Biochem. 2008;77:521-55. DOI:10.1146/annurev.biochem.76.061005.092322 |