Co-repressor induced order and biotin repressor dimerization: a case for divergent followed by convergent evolution

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Biotin enzyme BirA catalyzes biotinylation of biotin carboxyl carrier protein (BCCP).
• Biotin-5'-AMP acts as a co-repressor, inducing BirA dimerization and repressing biotin biosynthesis.
• Previous crystal structures of apo-BirA and BirA-biotin complex are known.

Purpose of the Study:

• To determine the 2.8Å resolution crystal structure of BirA in complex with the co-repressor analog, biotinol-5'-AMP.
• To elucidate the structural basis for co-repressor-mediated BirA dimerization and DNA binding.
• To understand the role of the nucleotide-binding motif in BirA's regulatory function.

Main Methods:

• X-ray crystallography (2.8Å resolution).
• Structural analysis and comparison of apo-BirA, BirA-biotin, and BirA-biotinol-5'-AMP complexes.

Main Results:

• The co-repressor complex adopts a dimeric structure with significant conformational changes compared to the BirA-biotin complex.
• A disordered adenylate binding loop becomes structured, covering the co-repressor and stabilizing the dimer interface.
• The co-repressor binding induces a 12-degree change in the hinge-bending angle, optimizing BirA binding to the bio operator.
• An unexpected role for the GXGXXG motif in adenylate binding and repressor function was revealed.

Conclusions:

• The structure of the BirA-biotinol-5'-AMP complex explains the co-repressor's role in optimizing BirA-DNA interaction.
• The findings highlight an unusual function of the GXGXXG motif in regulating enzyme activity.
• Structural analysis suggests early divergence of adenylating enzymes and independent evolution of adenylation activity.