Using concatenated subunits to investigate the functional consequences of heterotetrameric inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP 3 Rs) are a family of ubiquitous, ER localized, tetrameric Ca 2+ release channels. There are three subtypes of the IP 3 Rs (R1, R2, R3), encoded by three distinct genes, that share ∼60–70% sequence identity. The diversity of Ca 2+ signals generated by IP 3 Rs is thought to be largely the result of differential tissue expression, intracellular localization and subtype-specific regulation of the three subtypes by various cellular factors, most significantly InsP 3 , Ca 2+ and ATP. However, largely unexplored is the notion of additional signal diversity arising from the assembly of both homo and heterotetrameric InsP 3 Rs. In the present article, we review the biochemical and functional evidence supporting the existence of homo and heterotetrameric populations of InsP 3 Rs. In addition, we consider a strategy that utilizes genetically concatenated InsP 3 Rs to study the functional characteristics of heterotetramers with unequivocally defined composition. This approach reveals that the overall properties of IP 3 R are not necessarily simply a blend of the constituent monomers but that specific subtypes appear to dominate the overall characteristics of the tetramer. It is envisioned that the ability to generate tetramers with defined wild type and mutant subunits will be useful in probing fundamental questions relating to IP 3 R structure and function.