Role of aromatic residues at the lipid-water interface in micelle-bound bacteriophage M13 major coat protein

Area of Science:

• Biochemistry
• Structural Biology
• Membrane Biophysics

Background:

• Aromatic residues are observed to aggregate near the lipid-water interface in transmembrane protein domains.
• Understanding the role of these interfacial residues in protein-membrane interactions is crucial for deciphering membrane protein function.

Purpose of the Study:

• To investigate the role of interfacial tyrosine residues (Tyr-21 and Tyr-24) in the major coat protein of bacteriophage M13 (a model single membrane-spanning protein).
• To assess how mutations in these tyrosine residues impact protein-micelle interactions and structural stability.

Main Methods:

• Generation of a viable mutant library of the M13 major coat protein with specific mutations at Tyr-21 and Tyr-24.
• Utilizing intrinsic fluorescence of tryptophan (Trp-26) to probe changes in the micelle environment of mutants.
• Employing incremental-heating circular dichroism studies to determine alpha-helix to beta-structure transition temperatures.
• Performing site-directed and saturation mutagenesis experiments with phage viability as the assay.

Main Results:

• Polar mutations (e.g., Y24D, Y24N) resulted in reduced burial within deoxycholate micelles compared to wild-type protein.
• Polar mutants exhibited lower transition temperatures, indicating decreased structural stability.
• Nonpolar mutations and phenylalanine substitution (Y21F) showed increased stability, with Y21F being the most effective interfacial anchor.
• Simultaneous knockout of both tyrosine residues was not viable, highlighting their essentiality.

Conclusions:

• Specific side chains at the lipid-water interface significantly influence local protein-micelle interactions and protein stability.
• Interfacial tyrosine residues are critical for precise in vivo strand positioning, a function not optimally replaced by aliphatic residues.