Both the amino and carboxyl termini of dictyostelium myosin essential light chain are required for binding to myosin heavy chain

Dictyostelium myosin deficient in the essential light chain (ELC) does not function normally either in vivo or in vitro (Pollenz, R. S., Chen, T. L., Trivinos-Lagos, L., and Chisholm, R. L. (1992) Cell 69, 951-962). Since normal myosin function requires association of ELC, we investigated the domains of ELC that are necessary for binding to the myosin heavy chain (MHC). Deleting the NH₂-terminal 11 or 28 amino acid residues (ΔN11 or ΔN28) or the COOH-terminal 15 amino acid residues (ΔC15) abolished binding of the ELC to the MHC when the mutants were expressed in wild-type (WT) cells. In contrast, the ELC carrying deletion or insertion of four amino acid residues (D4 or I4) in the central linker segment bound the MHC in WT cells, although less efficient competition with WT ELC suggested that the affinity for the MHC is reduced. When these mutants were expressed in ELC-minus (mlcE^-) cells, where the binding to the heavy chain is not dependent on efficient competition with the endogenous ELC, ΔN28 and ΔN11 bound to the MHC at 15% of WT levels and ΔC15 did not bind to a significant degree. I4 and D4, however, bound with normal stoichiometry. These data indicate that residues at both termini of the ELC are required for association with the MHC, while the central linker domain appears to be less critical for binding. When the mutants were analyzed for their ability to complement the cytokinesis defect displayed by mice cells, a correlation to the level of ELC carried by the MHC was observed, indicating that a stoichiometric ELC-MHC association is necessary for normal myosin function in vivo.