Functional divergence prediction from evolutionary analysis: a case study of vertebrate hemoglobin

Area of Science:

• Evolutionary biology
• Bioinformatics
• Structural biology

Background:

• Gene duplication is a primary driver of novel protein functions.
• In silico methods analyze homologous sequences in paralogous proteins to predict protein function.
• Heterotachous sites (showing substitution rate shifts) have been proposed as indicators of functional divergence.

Purpose of the Study:

• To investigate signatures of functional specialization between paralogous protein subunits.
• To evaluate the utility of heterotachous sites versus a novel metric, "constant but different" (CBD) sites, in identifying functional divergence.
• To analyze the location of identified sites within the 3D structure of hemoglobin.

Main Methods:

• Comparative analysis of alpha and beta hemoglobin subunit sequences.
• Identification and quantification of heterotachous sites.
• Identification and quantification of "constant but different" (CBD) sites.
• Mapping of CBD sites onto the 3D structure of hemoglobin.

Main Results:

• Heterotachous sites were found in similar numbers between paralogous (alpha/beta) and orthologous (alpha/alpha, beta/beta) comparisons, questioning their role as specific divergence markers.
• CBD sites were significantly overrepresented in paralogous (alpha/beta) comparisons compared to orthologous ones.
• CBD sites were preferentially located on inter-subunit surfaces in the 3D structure of hemoglobin.

Conclusions:

• Substitution rate shifts (heterotachous sites) may reflect general protein evolution rather than specific functional specialization.
• "Constant but different" (CBD) sites are strong indicators of functional specialization between protein subunits.
• CBD site analysis, combined with structural data, offers a promising approach for characterizing multigene families.