The point mutation A34F causes dimerization of GB1.

Area of Science:

• Protein folding and design
• Structural biology
• Biophysics

Background:

• Streptococcal protein G (GB1) is a stable, small, single-domain protein widely used in protein folding and design studies.
• Previous GB1 variants have shown alternative folds, including intertwined tetramers and domain-swapped dimers.

Purpose of the Study:

• To determine the NMR structure of the single amino acid mutant Ala-34-Phe (A34F) of GB1.
• To characterize the dimerization behavior and interface of the A34F mutant.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy to determine the three-dimensional structure.
• Biochemical assays to measure dimer dissociation constant.

Main Results:

• The A34F mutant forms a side-by-side dimer with a dissociation constant of 27 +/- 4 microM.
• The dimer interface involves antiparallel beta-sheet pairing and a shortened alpha-helix forming intermolecular contacts.
• Significant differences from wild-type GB1 include alpha-helix dissolution and slow conformational motion of the interface loop, involving Tyr-33 side chain rearrangement.

Conclusions:

• The A34F mutation induces a novel side-by-side dimerization mode in GB1.
• The observed structural changes highlight the plasticity of GB1 and provide insights into protein folding pathways and dimer formation.