Smooth muscle myosin light chain kinase (MLCK) plays important roles in contractile-motile processes of a variety of cells. Three DFRxxL motifs at the kinase N-terminus (residues 2–63) are critical for high-affinity binding to actin-containing filaments [Smith et al. (1999) J. Biol. Chem. 274, 29433–29438]. A GST fusion protein containing residues 1–75 of MLCK (GST75-MLCK) bound maximally to both smooth muscle myofilaments and F-actin at 0.28 and 0.31 mol GST75-MLCK/mol actin with respective K D values of 0.1 μM and 0.8 μM. High-affinity binding of MLCK to actin-containing filaments may be due to each DFRxxL motif binding to one actin monomer in filaments.
