Evolution towards simplicity in bacterial small heat shock protein system.

Area of Science:

• Molecular biology
• Evolutionary biology
• Biochemistry

Background:

• Evolution can simplify multi-protein machines into single-protein systems with equivalent functions.
• The molecular mechanisms underlying such evolutionary simplification remain unclear.
• Bacterial small heat shock proteins (sHsp) provide a model to study this process.

Purpose of the Study:

• To investigate the evolutionary path of bacterial small heat shock proteins (sHsp) from a two-protein system to a single-protein system.
• To understand the molecular mechanisms enabling a single sHsp to perform functions previously requiring two distinct proteins (IbpA and IbpB).

Main Methods:

• Ancestral reconstruction
• Biochemical analysis
• Protein interaction studies
• Mutational analysis of α-crystallin domain

Main Results:

• Secondarily single sHsp evolved from IbpA, characterized by altered substrate binding and higher-order oligomerization.
• Two specific mutations in the α-crystallin domain reduced substrate binding affinity but enhanced refolding efficiency.
• Loss of IbpB function led to IbpA adopting its role by weakening substrate interaction, facilitating dissociation for refolding.
• The identified mutations dictate whether IbpA functions as a single sHsp or requires IbpB cooperation.

Conclusions:

• Evolutionary simplification of sHsp involved functional compensation and adaptation of substrate binding properties.
• Weakened substrate binding, driven by specific mutations, paradoxically enhances protein refolding by facilitating dissociation.
• This study elucidates the molecular basis for the evolution of a single protein performing the functions of two ancestral proteins.