MLN alters SERCA1a and SERCA2a kinetics but does not alter the apparent coupling ratio

The sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) are responsible for inducing muscle relaxation and are integral to the maintenance of intracellular calcium (Ca2+) homeostasis. As such, their activity is modulated by multiple regulatory proteins. Myoregulin (MLN) is a newly discovered protein inhibitor of SERCAs that physically interacts with the pump to regulate Ca2+-handling in muscle. While MLN has emerged as a key regulator of Ca2+ homeostasis and muscle contractility, it is unknown whether MLN can uncouple Ca2+ transport from ATP hydrolysis by SERCAs and in doing so, alter SERCAs’ apparent coupling ratio. To that end, HEK-293 cells were co-transfected with cDNA encoding SERCA1a alone or with SERCA1a and MLN and SERCA2a alone or with SERCA2a and MLN. Both Ca2+ uptake and Ca2+-ATPase activity were measured on crude cell homogenate prepared from the transfected cells. Ca2+-dependent SERCA activity was assessed over Ca2+ concentrations ranging from pCa 6.85 to 4.80 in presence and absence of the Ca2+ ionophore A23187 using a spectrophotometric plate reader assay. SERCA-mediated Ca2+ uptake was measured in the presence and absence of the precipitating anion, oxalate, using the fluorescent dye Indo-1 and a fluorometer. SERCA coupling ratio was calculated by dividing Ca2+ uptake by Ca2+-ATPase rates across different pCa values. In both the presence (p < 0.05) and absence of ionophore (p < 0.05), MLN significantly depressed the maximal rate of ATP consumption (VMAX) and SERCA’s Ca2+ affinity, with this effect being more pronounced in the presence of ionophore. Similarly, MLN significantly reduced SERCA Ca2+ uptake in both conditions, with a greater effect in the presence of oxalate. The ability of MLN to reduce VMAX and impede Ca2+ uptake was greater for SERCA1a compared to SERCA2a. These results indicate that MLN does not affect the Ca2+/ATP coupling ratio of SERCA1a and SERCA2a pumps at maximal (presence of ionophore/oxalate) and physiological (absence of ionophore/oxalate) conditions. NSERC This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.