The Dictyostelium essential light chain is required for myosin function.

A Dictyostelium mutant (7-11) that expresses less than 0.5% of wild-type levels of the myosin essential light chain (EMLC) has been created by overexpression of antisense RNA. Cells from 7-11 contain wild-type levels of the myosin heavy chain (MHC) and regulatory light chain (RMLC). Myosin isolated from 7-11 cells consists of the MHC with the RMLC associated in reduced stoichiometry, and binds to purified actin in an ATP-sensitive fashion. Purified 7-11 myosin displays calcium-activated ATPase activity with a Vmax about 15%-25% of that of wild type, and a Km for ATP of 27 +/- 5 microM versus 83 +/- 30 microM for wild type. At actin concentrations as high as 17 microM, 7-11 myosin displays greatly reduced actin-activated ATPase activity. Phenotypically, 7-11 cells resemble MHC mutants, growing poorly in suspension and becoming large and multinucleate. When starved for multicellular development, 7-11 cells take several hours longer than wild-type cells to aggregate. Although multicellular aggregates eventually form, they fail to develop further. The cells are also unable to cap receptors in response to Con A treatment. Since cells expressing the EMLC are phenotypically similar to MHC null mutants, the EMLC appears necessary for myosin function, at least in part because it is required for normal actin-activated ATPase activity.