A Nonsynonymous/Synonymous Substitution Analysis of the B56 Gene Family Aids in Understanding B56 Isoform Diversity

Gene duplication leads to the formation of gene families, wherein purifying or neutral selection maintains the original gene function, while diversifying selection confers new functions onto duplicated genes. The B56 gene family is highly conserved; it is encoded by one gene in protists and fungi, and five genes in vertebrates. B56 regulates protein phosphatase 2A (PP2A), an abundant heterotrimeric serine/threonine phosphatase that functions as a tumor suppressor and consists of a scaffolding "A" and catalytic "C" subunit heterodimer bound to a regulatory "B" subunit. Individual regulatory B56 subunits confer disparate functions onto PP2A in various cell-cell signaling pathways. B56 proteins share a conserved central core domain, but have divergent N- and C-termini which play a role in isoform specificity. We carried out a nonsynonymous/synonymous substitution analysis to better understand the divergence of vertebrate B56 genes. When five B56 paralogs from ten vertebrate species were analyzed, the gene family displayed purifying selection; stronger purifying selection was revealed when individual B56 isoforms were analyzed separately. The B56 core experienced stronger purifying selection than the N- and C-termini, which correlates with the presence of several contacts between the core and the AC heterodimer. Indeed, the majority of the contact points that we analyzed between B56 and the AC heterodimer experienced strong purifying selection. B56 subfamilies showed distinct patterns of selection in their N- and C-termini. The C-terminus of the B56-1 subfamily and the N-terminus of the B56-2 subfamily exhibited strong purifying selection, suggesting that these termini carry out subfamily-specific functions, while the opposite termini exhibited diversifying selection and likely carry out isoform-specific functions. We also found reduced synonymous substitutions at the N- and C-termini when grouping B56 genes by species but not by isoform, suggesting species-specific codon bias may have a role in regulating B56 gene expression.