MmuEX0044726 @ mm10

In vitro studies of recombinant alphaII-spectrin polypeptides representing the two splice variants of alphaII-spectrin, alphaII-cardi+ (with MmuEX0044726) and alphaII-cardi_ (without MmuEX0044726), revealed that the alphaII-cardi+ subunit has lower affinity for the complementary site in repeats 1-4 of betaII-spectrin, with a KD value of ~1 nM, as measured by surface plasmon resonance (SPR). In addition, the alphaII-cardi+ form showed 1.8-fold lower levels of binding to its site on betaII-spectrin than the alphaII-cardi_ form, both by SPR and blot overlay. This suggests that the 21-amino acid insert prevented some of the alphaII-cardi+ form from interacting with betaII-spectrin. Fusion proteins expressing the alphaII-cardi+ sequence within the two terminal spectrin repeats of alphaII-spectrin were insoluble in solution and aggregated in neonatal myocytes, consistent with the possibility that this insert removes a significant portion of the protein from the population that can bind beta subunits. Neonatal rat cardiomyocytes infected with adenovirus encoding GFP-fusion proteins of repeats 18-21 of alphaII-spectrin with the cardi+ insert formed many new processes. These processes were only rarely seen in myocytes expressing the fusion protein lacking the insert or in controls expressing only GFP. These results suggest that the embryonic mammalian heart expresses a significant amount of alphaII-spectrin with a reduced avidity for beta-spectrin and the ability to promote myocyte growth.