Structural and Functional Characterization of an Ancient
Bacterial Transglutaminase Sheds Light on the Minimal Requirements
for Protein Cross-Linking
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Abstract
Transglutaminases are best known
for their ability to catalyze
protein cross-linking reactions that impart chemical and physical
resilience to cellular structures. Here, we report the crystal structure
and characterization of Tgl, a transglutaminase from the bacterium <i>Bacillus subtilis.</i> Tgl is produced during sporulation and
cross-links the surface of the highly resilient spore. Tgl-like proteins
are found only in spore-forming bacteria of the <i>Bacillus</i> and <i>Clostridia</i> classes, indicating an ancient origin.
Tgl is a single-domain protein, produced in active form, and the smallest
transglutaminase characterized to date. We show that Tgl is structurally
similar to bacterial cell wall endopeptidases and has an NlpC/P60
catalytic core, thought to represent the ancestral unit of the cysteine
protease fold. We show that Tgl functions through a unique partially
redundant catalytic dyad formed by Cys116 and Glu187 or Glu115. Strikingly,
the catalytic Cys is insulated within a hydrophobic tunnel that traverses
the molecule from side to side. The lack of similarity of Tgl to other
transglutaminases together with its small size suggests that an NlpC/P60
catalytic core and insulation of the active site during catalysis
may be essential requirements for protein cross-linking