Molecular evolution of a-kinase anchoring protein (AKAP)-7: implications in comparative PKA compartmentalization

Background: A-Kinase Anchoring Proteins (AKAPs) are molecular scaffolding proteins mediating the assembly of multi-protein complexes containing cAMP-dependent protein kinase A (PKA), directing the kinase in discrete subcellular locations. Splice variants from the AKAP7 gene (AKAP15/18) are vital components of neuronal and cardiac phosphatase complexes, ion channels, cardiac Ca2+ handling and renal water transport. Results: Shown in evolutionary analyses, the formation of the AKAP7-RI/RII binding domain (required for AKAP/PKA-R interaction) corresponds to vertebrate-specific gene duplication events in the PKA-RI/RII subunits. Species analyses of AKAP7 splice variants shows the ancestral AKAP7 splice variant is AKAP7 alpha, while the ancestral long form AKAP7 splice variant is AKAP7 gamma. Multi-species AKAP7 gene alignments, show the recent formation of AKAP7 delta occurs with the loss of native AKAP7 gamma in rats and basal primates. AKAP7 gene alignments and two dimensional Western analyses indicate that AKAP7 gamma is produced from an internal translation-start site that is present in the AKAP7 delta cDNA of mice and humans but absent in rats. Immunofluorescence analysis of AKAP7 protein localization in both rat and mouse heart suggests AKAP7 gamma replaces AKAP7 delta at the cardiac sarcoplasmic reticulum in species other than rat. DNA sequencing identified Human AKAP7 delta insertion-deletions (indels) that promote the production of AKAP7 gamma instead of AKAP7 delta. Conclusions: This AKAP7 molecular evolution study shows that these vital scaffolding proteins developed in ancestral vertebrates and that independent mutations in the AKAP7 genes of rodents and early primates has resulted in the recent formation of AKAP7 delta, a splice variant of likely lesser importance in humans than currently described.