Binding of the utmost N-terminus of essential myosin light chains (ELC) to actin slows down myosin motor function. In this study, we investigated the binding constants of two different human cardiac ELC isoforms with actin. We employed circular dichroism (CD) and surface plasmon resonance (SPR) spectroscopy to determine structural properties and protein-protein interaction of recombinant human atrial and ventricular ELC (hALC-1 and hVLC-1, respectively) with alpha-actin as well as alpha-actin with alanin-mutated ELC binding site (alpha-actin(ala3)) as control. CD spectroscopy showed similar secondary structure of both hALC-1 and hVLC-1 with high degree of alpha-helicity. SPR spectroscopy revealed that the affinity of hALC-1 to alpha-actin (K-D = 575 nM) was significantly (p < 0.01) lower compared with the affinity of hVLC-1 to alpha-actin (K-D = 186 nM). The reduced affinity of hALC-1 to alpha-actin was mainly due to a significantly (p < 0.01) lower association rate (k(on): 1018 M-1 s(-1)) compared with kon of the hVLC-1/alpha-actin complex interaction (2908 M-1 s(-1)). Hence, differential expression of ELC isoforms could modulate muscle contractile activity via distinct alpha-actin interactions. (C) 2014 Elsevier Inc. All rights reserved.