Sequence determinants of a conformational switch in a protein structure

Area of Science:

• Protein structure and function
• Molecular biology
• Biochemistry

Background:

• The Arc repressor is a dimeric transcription factor from bacteriophage P22.
• It belongs to the ribbon-helix-helix family, characterized by specific structural motifs.
• Wild-type Arc forms antiparallel beta-strands involving residues 9-14, creating a beta-ribbon structure.

Purpose of the Study:

• To investigate the conformational preferences of Arc repressor mutants at position 11.
• To determine how specific amino acid substitutions influence the switch between beta-ribbon and helical structures.
• To understand the role of side chain chemical properties in dictating protein fold preference.

Main Methods:

• Site-directed mutagenesis was used to introduce various amino acids (Gly, Ala, Val, Ile, Leu, Met, Phe, Tyr) at position 11.
• Structural analysis of mutant Arc proteins was performed.
• The study utilized a combination of wild-type and negatively designed sequence backgrounds to probe conformational stability.

Main Results:

• Mutants at position 11 adopted the wild-type beta-ribbon structure in stabilizing sequence contexts.
• These mutants shifted to an alternative helical structure when the wild-type fold was disfavored by negative design.
• The chemical properties of the side chain at position 11 were found to dictate the preferred conformation in an otherwise wild-type sequence.

Conclusions:

• The Arc repressor exhibits conformational plasticity, capable of adopting distinct structural folds.
• Amino acid identity at position 11 plays a critical role in modulating this conformational switch.
• Understanding these structural preferences provides insights into transcription factor regulation and protein folding dynamics.