Molecular basis for curvature formation in SepF polymerization

Area of Science:

• Structural biology
• Biochemistry
• Microbiology

Background:

• Protein self-assembly into curved structures is vital for cellular processes, including bacterial cell division.
• The septum-forming protein (SepF) in Bacillus subtilis polymerizes into uniform curvatures, regulating septum thickness.
• SepF polymerization involves distinct β-β and α-α interfaces, but the curvature formation mechanism remains unclear.

Purpose of the Study:

• To investigate the mechanism of curvature formation in SepF polymerization.
• To compare the structural differences between wild-type cyclic SepF and a linear mutant (G137 mutation on β-β interface).
• To elucidate the role of inter-unit interfaces in controlling protein assembly shape.

Main Methods:

• Solid-state Nuclear Magnetic Resonance (SSNMR) spectroscopy to analyze protein structures.
• Cryo-electron microscopy (cryo-EM) for high-resolution imaging of assemblies.
• Computational simulations to model and understand structural dynamics.

Main Results:

• Sequence variations within the assembly unit (due to G137 mutation) did not significantly alter assembly shape.
• The α-α interface, mediating contacts between assembly units, was identified as the primary determinant of assembly curvature.
• Atomic-level analysis revealed that the angular orientation of the α2 helix within the α-α interface controls assembly curvature.

Conclusions:

• The curvature of SepF assemblies is controlled by the inter-unit α-α interface, specifically the orientation of the α2 helix.
• Interhelical contacts are critical for maintaining the specific curvature of protein assemblies.
• This study provides atomic insights into protein self-assembly and shape regulation.