Nonmuscle myosin phosphorylation sites for calcium-dependent and calcium-independent protein kinases

Airway smooth muscle in asthma: linking contraction and mechanotransduction to disease pathogenesis and remodelling

Asthma is an obstructive airway disease, with a heterogeneous and multifactorial pathogenesis. Although generally considered to be a disease principally driven by chronic inflammation, it is becoming increasingly recognised that the immune component of the pathology poorly correlates with the clinical symptoms of asthma, thus highlighting a potentially central role for non-immune cells. In this context airway smooth muscle (ASM) may be a key player, as it comprises a significant proportion of the airway wall and is the ultimate effector of acute airway narrowing. Historically, the contribution of ASM to asthma pathogenesis has been contentious, yet emerging evidence suggests that ASM contractile activation imparts chronic effects that extend well beyond the temporary effects of bronchoconstriction. In this review article we describe the effects that ASM contraction, in combination with cellular mechanotransduction and novel contraction-inflammation synergies, contribute to asthma pathogenesis. Specific emphasis will be placed on the effects that ASM contraction exerts on the mechanical properties of the airway wall, as well as novel mechanisms by which ASM contraction may contribute to more established features of asthma such as airway wall remodelling.

Phosphorylation of myosin light chain kinase by p21-activated kinase PAK2

Phosphorylation of myosin II regulatory light chains (RLC) by Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) is a critical step in the initiation of smooth muscle and non-muscle cell contraction. Post-translational modifications to MLCK down-regulate enzyme activity, suppressing RLC phosphorylation, myosin II activation, and tension development. Here we report that PAK2, a member of the Rho family of GTPase-dependent kinases, regulates isometric tension development and myosin II RLC phosphorylation in saponin permeabilized endothelial monolayers. PAK2 blunts tension development by 75% while inhibiting diphosphorylation of myosin II RLC. Cdc42-activated placenta and recombinant, constitutively active PAK2 phosphorylate MLCK in vitro with a stoichiometry of 1.71 +/- 0. 21 mol of PO(4)/mol of MLCK. This phosphorylation inhibits MLCK phosphorylation of myosin II RLC. PAK2 catalyzes MLCK phosphorylation on serine residues 439 and 991. Binding calmodulin to MLCK blocks phosphorylation of Ser-991 by PAK2. These results demonstrate that PAK2 can directly phosphorylate MLCK, inhibiting its activity and limiting the development of isometric tension.

Phosphorylation of non-muscle myosin II regulatory light chain by p21-activated kinase (gamma-PAK)

Myosin regulatory light chain (RLC) phosphorylation has been implicated in Rho-mediated stress fibre formation. The recent observation that Rho kinase phosphorylates RLC in vitro suggests that serine/threonine kinases other than those in the myosin light chain kinase (MLCK) family have the potential to activate myosin II. In this study we report that gamma-PAK, which is activated by the GTP-binding proteins Cdc42 and Rac, catalyses phosphorylation of intact non-muscle myosin II and isolated recombinant RLC. gamma-PAK phosphorylated endothelial cell myosin II to 0.85 +/- 0.02 mol PO4 per mol RLC. Phosphorylation is Ca2+/calmodulin-independent and the enzyme has a K(m) and Vmax for myosin II regulatory light chain of 12 microM and 180 nmol/min/mg respectively. No myosin II heavy chain phosphorylation was detected. Phosphopeptide maps and phosphoamino acid analysis revealed that gamma-PAK phosphorylates Ser-19 but does not phosphorylate Thr-18. A panel of recombinant RLC mutants was used to confirm that Ser-19 is the only phosphorylation site modified by gamma-PAK. On substitution of both Ser-19 and Thr-18 with Ala or Glu, no phosphorylation of other Ser/Thr residues in the RLC was detected. Similar to MLCK, Arg-16 is required for interaction of gamma-PAK with the substrate, since converting Arg-16 to Ala significantly reduced RLC phosphorylation. Endothelial cell monolayers permeabilized with saponin retract upon exposure to either Cdc42 or trypsin-activated gamma-PAK and ATP. Activation of gamma-PAK is required to initiate Ca2+/calmodulin-independent cell retraction and actin rearrangement. Taken together, these data suggest that myosin II activation by the p21-activated family of kinases may be physiologically important in regulating cytoskeletal organization.

Site-specific phosphorylation of a synthetic peptide derived from ribosomal protein S6 by human placenta protein kinases

A synthetic peptide S6-21 (AKRRRLSSLRASTSKSESSQK) which contains the phosphorylated residues in the ribosomal protein S6 has been used as a substrate for two partially purified human placenta protein kinases. Two distinct classes of protein kinases which catalyze either amino terminal (AKRRRLSS) or carboxyl terminal (LRASTSKSESSQK) peptide phosphorylation were identified. Multiple sites were phosphorylated in each domain. A single protein kinase which catalyzed phosphorylation of sites in both domains was identified. Although growth factors are known to promote phosphorylation of S6 at five serine sites, no enzyme which could modify S6-21 to that extent was observed.

Use of DNA sequence and mutant analyses and antisense oligodeoxynucleotides to examine the molecular basis of nonmuscle myosin light chain kinase autoinhibition, calmodulin recognition, and activity

The first primary structure for a nonmuscle myosin light chain kinase (nmMLCK) has been determined by elucidation of the cDNA sequence encoding the protein kinase from chicken embryo fibroblasts, and insight into the molecular mechanism of calmodulin (CaM) recognition and activation has been obtained by the use of site-specific mutagenesis and suppressor mutant analysis. Treatment of chicken and mouse fibroblasts with antisense oligodeoxynucleotides based on the cDNA sequence results in an apparent decrease in MLCK levels, an altered morphology reminiscent of that seen in v-src-transformed cells, and a possible effect on cell proliferation. nmMLCK is distinct from and larger than smooth muscle MLCK (smMLCK), although their extended DNA sequence identity is suggestive of a close genetic relationship not found with skeletal muscle MLCK. The analysis of 20 mutant MLCKs indicates that the autoinhibitory and CaM recognition activities are centered in distinct but functionally coupled amino acid sequences (residues 1,068-1,080 and 1,082-1,101, respectively). Analysis of enzyme chimeras, random mutations, inverted sequences, and point mutations in the 1,082-1,101 region demonstrates its functional importance for CaM recognition but not autoinhibition. In contrast, certain mutations in the 1,068-1,080 region result in a constitutively active MLCK that still binds CaM. These results suggest that CaM/protein kinase complexes use similar structural themes to transduce calcium signals into selective biological responses, demonstrate a direct link between nmMLCK and non-muscle cell function, and provide a firm basis for genetic studies and analyses of how nmMLCK is involved in development and cell proliferation.

Isolation and characterization of protein kinase C from Y-1 adrenal cell cytoskeleton

The cytoskeletons of Y-1 mouse adrenal tumor cells contain a calcium and phospholipid-dependent protein kinase (protein kinase C) that is bound sufficiently tight to resist extraction by 0.5% Triton but not by 1.0% Triton. The enzyme has been purified to near homogeneity from cytoskeleton and cytosol. It shows features typical of this type of kinase, namely a requirement for Ca2+ and phospholipid, stimulation by tumor promoters but not by nontumor-promoting phorbol esters, and inhibition by trifluoperazine. The enzyme shows specificity for four substrates found in the cytoskeleton, namely 80, 33, 20, and 18 kD. The first three substrates are phosphorylated by the enzyme; the fourth is dephosphorylated and is therefore affected by the kinase indirectly. The 80-kD protein is the kinase enzyme itself which is autophosphorylated in vitro and in the cytoskeleton. The 20-kD protein is myosin light chain. The 33- and 18-kD proteins are unidentified. The same substrates were phosphorylated when Y-1 cells were permeabilized with digitonin and incubated with [gamma-32P]ATP and phorbol-12-myristate-13-acetate. Partly purified protein kinase C changes the extent of phosphorylation of the same substrates when added to cytoskeletons previously extracted to remove endogenous protein kinase C. Addition of Ca2+, phosphatidylserine, and phorbol-12-myristate-13-acetate to cytoskeletons, and addition of these three agents plus protein kinase C to extracted cytoskeletons, causes these structures to undergo a rapid and extensive rounding. A similar change is induced in intact cells by addition of phorbol ester. It is concluded that protein kinase C is capable of changing the shape of adrenal cells by an action that involves autophosphorylation and phosphorylation of myosin light chain. This response may in turn be related to the steroidogenic responses to ACTH and cyclic AMP.

Synthetic peptides derived from the nonmuscle myosin light chains are highly specific substrates for protein kinase C

The phosphorylation of synthetic peptides derived from the NH2-terminal sequence of smooth-muscle myosin was studied with purified protein kinase C. The protein kinase C phosphorylation domain included both serine residues and threonine residues in the sequence SSKRAKAKTTKKR(G), denoted myosin light chain (1-13) (MLC(1-13)). Kinetic analysis of MLC(1-13) and truncated peptides derived from the parent peptide established that removal of the serine residues had little effect on protein kinase C reactivity. MLC(1-13) had a V/K of 2.4 min-1.mg-1, whereas the V/K of MLC(3-13) was 3.0 min-1.mg-1. Removal of Lys-3 resulted in a 50% decrease in V/K which was attributable to a 50% decrease in apparent Vmax.Arg-4 was established as a significant protein kinase C specificity determinant, since the apparent Km increased 7-fold and the Vmax decreased 3-fold when the parent peptide was truncated at that residue. All peptides studied required calcium and lipid effectors for full activity with protein kinase C, indicating that they are Class C substrates as defined by Bazzi and Nelsestuen (Biochemistry 26 (1987) 5002) for protein kinase C. Other protein kinases, including cyclic AMP- and cyclic GMP-dependent protein kinase, S6/H4 kinase, myosin light-chain kinase and calcium/calmodulin-dependent kinase II, had little or no activity with these peptides. In studies on the purification of lymphosarcoma protein kinase C by several chromatographic procedures, the results showed that the myosin light-chain peptides can provide convenient and well-characterized substrates for purification and mechanistic studies of protein kinase C biochemistry.