Activation of the cardiac Na+-Ca2+ exchanger by sorcin via the interaction of the respective Ca2+-binding domains

Sorcin is a penta-EF-hand protein that Interacts with intracellular target proteins after Ca2+ binding The sarcolemmal Na+/Ca2+ exchanger (NCX1) may be an important sorcin target in cardiac muscle In this study, RNAi knockdown of sorcin, purified sorcin or sorcin variants was employed in parallel measurements of. (i) NCX activity in isolated rabbit cardiomyocytes using electrophysiological techniques and (ii) sorcin binding to the NCX1 calcium binding domains (CBD1 and (in) using surface plasmon resonance and gel overlay techniques Sorcin is activated by Ca2+ binding to the EF3 and EF2 regions, which are connected by the D helix To investigate the importance of this region in the interaction with NCX1, three variants were examined. W105G and W99G, mutated respectively near EF3 and EF2, and E124A that does not bind Ca2+-due to a mutation at EF3. Downregulation of sorcin decreased and supplementation with wt sorcin (3 mu M) increased NCX activity in Isolated cardiomyocytes The relative stimulatory effects of the sorcin variants were W105G>wt sorcin>Sorcin Calcium Binding Domain (SCBD)>W99G>E124A. Sorcin binding to both CBD1 and 2 was observed. In the presence of 50 mu M Ca2+, the interaction with CBD1 followed the order W105G>SCBD>wt sorcin>W99G>E124A. In sorcin, the interacting surface can be mapped on the C-terminal Ca2+-binding domain in the D helix region comprising W99 The fast association/dissociation rates that characterize the interaction of sorcin with CBD1 and 2 may permit complex formation/dissociation during an excitation/contraction cycle (C) 2010 Elsevier Ltd All rights reserved