[Show abstract][Hide abstract] ABSTRACT: Short and long myosin light chain kinases (MLCKs) are Ca2+/calmodulin-dependent enzymes that phosphorylate the regulatory light chain of myosin II in thick filaments but bind with
high affinity to actin thin filaments. Three repeats of a motif made up of the sequence DFRXXL at the N terminus of short MLCK are necessary for actin binding (Smith, L., Su, X., Lin, P., Zhi, G., and Stull, J. T. (1999)
J. Biol. Chem. 274, 29433–29438). The long MLCK has two additional DFRXXL motifs and six Ig-like modules in an N-terminal extension, which may confer unique binding properties for cellular localization.
Two peptides containing either five or three DFRXXL motifs bound to F-actin and smooth muscle myofilaments with maximal binding stoichiometries consistent with each motif binding
to an actin monomer in the filaments. Both peptides cross-linked F-actin and bound to stress fibers in cells. Long MLCK with
an internal deletion of the five DFRXXL motifs and the unique NH2-terminal fragment containing six Ig-like motifs showed weak binding. Cell fractionation and extractions with MgCl2 indicate that the long MLCK has a greater affinity for actin-containing filaments than short MLCK in vitro andin vivo. Whereas DFRXXL motifs are necessary and sufficient for short MLCK binding to actin-containing filaments, the DFRXXL motifs and the N-terminal extension of long MLCK confer high affinity binding to stress fibers in cells.
Preview · Article · Oct 2002 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Ca2+-calmodulin-dependent phosphorylation of myosin regulatory light chains by the catalytic COOH-terminal half of myosin light chain kinase (MLCK) activates myosin II in smooth and nonmuscle cells. In addition, MLCK binds to thin filaments in situ and F-actin in vitro via a specific repeat motif in its NH2 terminus at a stoichiometry of one MLCK per three actin monomers. We have investigated the structural basis of MLCK-actin interactions by negative staining and helical reconstruction. F-actin was decorated with a peptide containing the NH2-terminal 147 residues of MLCK (MLCK-147) that binds to F-actin with high affinity. MLCK-147 caused formation of F-actin rafts, and single filaments within rafts were used for structural analysis. Three-dimensional reconstructions showed MLCK density on the extreme periphery of subdomain-1 of each actin monomer forming a bridge to the periphery of subdomain-4 of the azimuthally adjacent actin. Fitting the reconstruction to the atomic model of F-actin revealed interaction of MLCK-147 close to the COOH terminus of the first actin and near residues 228-232 of the second. This unique location enables MLCK to bind to actin without interfering with the binding of any other key actin-binding proteins, including myosin, tropomyosin, caldesmon, and calponin.
Full-text · Article · Sep 2001 · The Journal of Cell Biology
[Show abstract][Hide abstract] ABSTRACT: 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 microM and 0.8 microM. High-affinity binding of MLCK to actin-containing filaments may be due to each DFRxxL motif binding to one actin monomer in filaments.