Recent work on molecular genetics of mammalian contractile proteins has provided valuable insights into the basis of the heterogeneity of muscle proteins and their regulated expression during development, yet information on the precise role(s) of light chains in actomyosin interaction and muscle function is still lacking. The selective increase in ventricular myosin light chain-2 (MLC2) in hypertrophied heart muscle has been implicated as a compensatory feature of myosin, but its relevance to myosin function is not known. To investigate the role of cardiac MLC2, we have isolated a full-length cDNA clone for human ventricular MLC2 and produced a full-length and N-terminal deleted MLC2 polypeptides in Escherichia coli using the bacterial expression vector pT7-7 system. The interaction of recombinant MLC2 with myosin heavy chain (MHC) and its subfragment-1 was studied using the full-length and truncated recombinant polypeptides. The results demonstrated that the bacterially produced full-length human cardiac MLC2 exchanges effectively with the native MLC2 and binds with specificity to MHC and to intact myofibrils. Domain mapping by deletion and in vitro exchange/competition analysis with a synthetic peptide suggests that a conserved central domain in MLC2 participates in the functional association of the two myosin subunits.