Initiation and propagation of spectrin heterodimer assembly involves distinct energetic processes

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biophysics

Background:

• Red cell spectrin alpha and beta subunits assemble into heterodimers through tandem homologous motifs.
• Dimerization requires a specific 'dimer initiation' site, but subsequent interactions are less understood.

Purpose of the Study:

• To elucidate the mechanism and energetics of the two heterodimer assembly phases in spectrin.
• To investigate the forces driving both the high-affinity initiation and low-affinity lateral interactions.

Main Methods:

• Isothermal titration calorimetry (ITC) was employed to measure the thermodynamic parameters of heterodimer assembly.
• Analysis of proton exchange was used to identify specific interactions involving pKa shifts.

Main Results:

• High-affinity dimer initiation is enthalpically driven, involving electrostatic and hydrophilic interactions at the initiation site.
• Subsequent lateral associations are entropically driven, indicating hydrophobic interactions.
• A specific interaction between alpha18 and beta4 repeats shows a net proton uptake, potentially due to a histidine pKa shift.

Conclusions:

• Spectrin heterodimer assembly proceeds in two distinct phases with different thermodynamic driving forces.
• A detailed model of spectrin dimer assembly is proposed based on the thermodynamic analysis.
• Specific motif interactions, like alpha18-beta4, contribute uniquely to the assembly process.