The cytoplasmic domain of small Ankyrin 1 binds to SERCA1 to regulate SERCA1’s activity in skeletal muscle

Abstract

SERCA1 is a Ca2+ pump that is responsible for cytoplasmic Ca2+ clearance during muscle relaxation. Recently, small ankyrin 1 (sAnk1) was identified as a novel regulator of SERCA1. In our previous study, we showed that sAnk1 interacts directly with SERCA1 via its transmembrane and cytoplasmic domains. This work focuses on characterizing the binding between the cytoplasmic domain of sAnk1 (sAnk1(29-155)) and SERCA1 and investigating the physiological consequences of this interaction. To date, we have identified 2 regions in sAnk1(29-155) that mediate binding to SERCA1, which are V29-H41 and R64- K73. A series of mutations were made in each regions and the mutants were tested for binding to SERCA1 via blot overlay assays, to identify the effect of charged/hydrophobic residues on the binding. The CD spectra showed that, among all mutants that interrupted binding to SERCA1, only mutants that has alanine mutations in 38K/39H/41H maintained the same folding as wild type sAnk1(29-155). We also found that the F3 region in the C- terminus of obscurin (Obsc-F3), which binds to sAnk1, also binds directly to SERCA1 via hydrophobic interactions. In tests of affinity, Obsc-F3 binds to SERCA1 with an affinity that is several-fold weaker than binding by sAnk1(29-155). In coimmunoprecipitation experiments, Obsc-F3 and sAnk1(29-155) bind to the same sites on SERCA1, but they can also bind to SERCA1 simultaneously, presumably because they can still bind to each other while bound to SERCA1. Consistent with this, the binding site for Obsc-F3 on sAnk1(29-155) is not the same as the site on sAnk1(29-155) that binds SERCA1. Our working hypothesis is that the direct binding of sAnk1(29-155) to SERCA1 not only alters SERCA1’s function directly (which we showed before) but also serves as a platform for other proteins, such as obscurin, to regulate SERCA1’s activity. To test our hypothesis, we will study the ATPase activity of SERCA1 when complexed with sAnk1(29-155), obsc-F3, and both protein fragments, and the effects of manipulating the interactions between sAnk1(29-155) and SERCA1 in cells and in isolated muscle fibers.