A Folding Insulator Defines Cryptic Domains in Tropomyosin

Area of Science:

• Protein structure and folding
• Molecular evolution
• Biophysics

Background:

• Multidomain proteins evolve through modular assembly, often exhibiting distinct folding transitions.
• Tropomyosin, a continuous alpha-helix involved in muscle and cytoskeletal filaments, lacks apparent domain boundaries.
• Despite its continuous structure, tropomyosin's unfolding suggests the presence of multiple folding domains.

Purpose of the Study:

• To investigate how structurally cryptic domains can exist within a continuous protein structure.
• To model the folding behavior of proteins like tropomyosin with potential hidden domains.
• To correlate protein structure and folding with evolutionary mechanisms and disease.

Main Methods:

• Utilized Lifson-Roig helix-coil models with varying helical nucleation propensities to simulate protein folding.
• Employed hydrogen-deuterium exchange mass spectrometry to identify the positions of folding domain boundaries.
• Correlated biophysical folding studies with theoretical modeling.

Main Results:

• Models with a central folding insulator best replicated experimental folding data for tropomyosin.
• Identified distinct N-terminal and C-terminal folding domains separated by an insulator.
• Hydrogen-deuterium exchange mass spectrometry confirmed the presence and location of these cryptic domains.

Conclusions:

• Structurally cryptic folding domains can exist within seemingly continuous protein structures like tropomyosin.
• These hidden domains may arise from evolutionary selection for functional diversity.
• The presence of these domains could explain the link between tropomyosin mutations and cardiomyopathies.