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Val-->Ala mutations selectively alter helix-helix packing in the transmembrane segment of phage M13 coat protein
1Division of Biochemistry Research, Hospital for Sick Children, Toronto, ON, Canada.
Abstract:
Val-->Ala mutations within the effective transmembrane segment of a model single-spanning membrane protein, the 50-residue major coat (gene VIII) protein of bacteriophage M13, are shown to have sequence-dependent impacts on stabilization of membrane-embedded helical dimeric structures. Randomized mutagenesis performed on the coat protein hydrophobic segment 21-39 (YIGYAWAMV-VVIVGATIGI) produced a library of viable mutants which included those in which each of the four valine residues was replaced by an alanine residue. Significant variations found among these Val-->Ala mutants in the relative populations and thermal stabilities of monomeric and dimeric helical species observed on SDS/PAGE, and in the range of their alpha-helix-->beta-sheet transition temperatures confirmed that intramembranous valine residues are not simply universal contributors to membrane anchoring. Additional analyses of (i) nonmutatable sites in the mutant protein library, (ii) the properties of the double mutant V29A-V31A obtained by recycling mutant V31A DNA through mutagenesis procedures, and (iii) energy-minimized helical dimer structures of wild-type and mutant V31A transmembrane regions indicated that the transmembrane hydrophobic core helix of the M13 coat protein can be partitioned into alternating pairs of potential protein-interactive residues (V30, V31; G34, A35; G38, I39) and membrane-interactive residues (M28, V29; I32, V33; T36, I37). The overall results consitute an experimental approach to categorizing the distinctive contributions to structure of the residues comprising a protein-protein packing interface vs. those facing lipid and confirm the sequence-dependent capacity of specific residues within the transmembrane domain to modulate protein-protein interactions which underlie regulatory events in membrane proteins.
Insights
Valine to alanine mutations in a model membrane protein reveal sequence-dependent stabilization of helical structures. Specific residues influence protein-protein interactions within the transmembrane domain, not just membrane anchoring.
Area of Science:
- Biochemistry
- Structural Biology
- Membrane Protein Dynamics
Background:
- The M13 bacteriophage coat protein is a model for single-spanning membrane proteins.
- Understanding transmembrane domain interactions is crucial for membrane protein function.
Purpose of the Study:
- To investigate the role of valine residues in the transmembrane segment of the M13 coat protein.
- To determine how sequence-specific mutations affect helical structure stabilization and protein-protein interactions.
Main Methods:
- Randomized mutagenesis of the M13 coat protein hydrophobic segment (residues 21-39).
- Analysis of Val-->Ala mutants using SDS/PAGE to assess helical species and thermal stability.
- Energy minimization of wild-type and mutant transmembrane helical dimer structures.
Main Results:
- Valine to alanine mutations exhibited sequence-dependent effects on helical dimer stabilization.
- Intramembranous valine residues differentially contribute to membrane anchoring and protein interactions.
- The transmembrane helix can be partitioned into distinct protein-interactive and membrane-interactive residue pairs.
Conclusions:
- Specific residues within the transmembrane domain modulate protein-protein interactions.
- The findings provide an experimental framework for categorizing residue contributions at protein-protein interfaces versus lipid-facing sites.
- Sequence-dependent residue roles are critical for regulating membrane protein interactions.