Serine substitutions are linked to codon usage and differ for variable and conserved protein regions

Area of Science:

• Molecular Biology
• Evolutionary Biology
• Genetics

Background:

• Serine is uniquely encoded by two distinct codon sets (TCN and AGY).
• Nucleotide substitutions between these serine codon sets require two changes.
• This unique encoding may influence amino acid substitution patterns under different selective pressures.

Purpose of the Study:

• To investigate how serine's dual codon sets affect substitution patterns in conserved versus variable protein regions.
• To explore the evolutionary implications of these distinct substitution patterns in human and viral proteins.
• To analyze the impact of codon usage on serine's phosphorylation capability in human exomes.

Main Methods:

• Comparative analysis of conserved and non-conserved positions in vertebrate proteins.
• Examination of substitution patterns in HIV envelope glycoprotein (gp120) and antibody V-genes.
• Analysis of human exome data to assess codon substitutions related to phosphorylation.

Main Results:

• Conserved human protein positions show enrichment for TCN-encoded serine substitutions (S'PA), while less conserved positions favor AGY-encoded serine (GS″N).
• This pattern was mirrored in conserved and hypervariable regions of HIV gp120 and antibody V-genes.
• Serine codons (S') involved in phosphorylation are significantly less prone to substitutions that could mimic phosphorylation capacity compared to S″ codons.

Conclusions:

• Serine's dual codon sets facilitate distinct substitution patterns, enabling functional maintenance in conserved regions and rapid diversification in variable regions.
• These findings reveal a novel mechanism by which codon usage shapes protein evolution and function.
• The differential substitution patterns highlight an evolutionary strategy for maintaining protein structure and function while allowing for adaptive change.