Myosin Light Chain Kinase Structure Function Analysis Using Bacterial Expression

Phosphorylation and activation of a transducible recombinant form of human HSP20 in Escherichia coli

Protein-based cellular therapeutics have been limited by getting molecules into cells and the fact that many proteins require post-translational modifications for activation. Protein transduction domains (PTDs), including that from the HIV TAT protein (TAT), are small arginine rich peptides that carry molecules across the cell membrane. We have shown that the heat shock-related protein, HSP20 is a downstream-mediator of cyclic nucleotide-dependent relaxation of vascular smooth muscle and is activated by phosphorylation. In this study, we co-expressed in Escherichia coli the cDNAs encoding the catalytic subunit of protein kinase G and a TAT-HSP20 fusion protein composed of the TAT PTD (-YGRKKRRQRRR-) fused to the N-terminus of human HSP20. Immunoblot and HPLC-ESI-MS/MS analysis of the purified TAT-HSP20 demonstrated that it was phosphorylated at serine 40 (equivalent to serine 16 in wild-type human HSP20). This phosphorylated TAT-HSP20 was physiologically active in intact smooth muscles in that it inhibited 5-hydroxytryptamine-induced contractions by 57%+/-4.5. The recombinant phosphorylated protein also led to changes in actin cytoskeletal morphology in 3T3 cells. These results delineate strategies for the expression and activation of therapeutic molecules for intracellular protein based therapeutics.

Calmodulin kinase II chimeras used to investigate the structural requirements for smooth muscle myosin light chain kinase autoinhibition and calmodulin-dependent activation

Segments of the autoregulatory domain of MK, a catalytically active fragment of the monomeric smooth muscle myosin light chain kinase (smMLCK) (residues 472-972), were replaced with their counterparts from a homologous but multimeric enzyme, calmodulin-dependent protein kinase II (CaM KII). Chimeric proteins in which both the autoregulatory and oligomerization domains of CaM KII (residues 281-478) were substituted for residues 781-972 of smMLCK, MK(CK281-478), or only the autoregulatory domain of CaM KII (residues 281-315) was exchanged for residues 781-813 of smMLCK, MK(CK281-315), exhibited significant enzymatic activity in the absence of Ca(2+)/CaM. In contrast, both MK and a chimeric protein in which the C-terminal half of the autoregulatory domain of smMLCK was replaced with CaM KII residues 301-315, MK(CK301-315), were inactive in the absence of Ca(2+)/CaM. These results indicate that the sequence of the N-terminal half of the autoregulatory domain of smMLCK is important for complete autoinhibition of its enzymatic activity. All proteins bound to Ca(2+)/CaM, and the chimeric proteins MK(CK281-478) and MK(CK281-315) were activated by Ca(2+)/CaM with activation constants (K(CaM)) and maximal enzymatic activities comparable to those of the wild-type MK enzyme. This demonstrates that the entire autoregulatory domain of CaM KII can replace that of smMLCK in its ability to promote efficient CaM-dependent activation of the smMLCK enzyme. However, the inability of the chimeric protein MK(CK301-315) to be activated by Ca(2+)/CaM suggests that replacement of only the C-terminal half of the autoregulatory domain of smMLCK, while still retaining the ability to bind Ca(2+)/CaM, also substitutes residues that prevent activation of the enzyme by Ca(2+)/CaM.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Nitric oxide blocks the cell cycle of mouse macrophage-like cells in the early G2+M phase

The effects of nitric oxide produced by macrophage-like cells (Mm1) on the cell cycle were investigated. Mm1 cells lost proliferative activity in the presence of interleukin-6 (IL-6) and a subpopulation accumulated in the G2+M phase. This level increased in proportion to the incubation time. The DNA content of the cells was slightly lower than that of Mm1 cells treated with vinblastine or demecolcine, drugs which block the cell cycle in the M phase. The peak of the early G2+M phase was reduced by treatment with NG-mono-methyl-L-arginine. However, after treatment with exogenous nitric oxide or sodium nitroprusside, the G0/G1 phase increased, but the early-G2+M and the S phase decreased. The flow cytometry pattern in IL-6-treated Mm1 was the same as that of cytochalasin B-treated Mm1. These data suggest that endogenous nitric oxide affects the microfilament system of IL-6-treated Mm1 cells and blocks the cell cycle in the early G2+M phase.

Expression of cGMP-dependent protein kinase in Escherichia coli

Cyclic GMP-dependent protein kinase (cGMP kinase) is involved in the relaxation of smooth muscle. The enzyme has been cloned and expressed in eukaryotic cell lines but so far not in prokaryotic cells. Three vectors were constructed for the expression of I alpha cGMP kinase in Escherichia coli. Transformation with the pET3a/cgk vector which uses the T7 RNA polymerase/promotor system resulted in efficient accumulation of cGMP kinase. Most of the protein was in an insoluble and catalytic inactive form. Various solubilization and refolding conditions did not yield an active enzyme. A small fraction of the cGMP kinase was present in the soluble cell extract. This fraction bound cGMP with high affinity but had no cGMP stimulated kinase activity. To prevent aggregation two additional vectors were constructed. (I) A bacterial leader sequence, which directs the export of proteins into the periplasmic space, was fused to the amino-terminus of the cGMP kinase. (II) A gram/gram+ shuttle vector for expression under the control of the tac promotor was used. Both constructs directed the synthesis of an insoluble and inactive cGMP kinase. These results suggest that large amounts of cGMP kinase can be expressed in E. coli, but mainly in an insoluble and inactive form. In contrast to eukaryotic cells, bacteria may lack systems for correct protein folding and/or posttranslational modification that are crucial for the productive folding and/or activation of cGMP kinase.

Molecular cloning of the chicken gizzard telokin gene and cDNA

Telokin is a protein which consiste of the C-terminal portion of smooth muscle myosin light chain kinase (MLCK) (M. Ito, R. Dabrowska, V. Guerriero, Jr., and D. J. Hartshone (1989) J. Biol. Chem. 264, 13971-13974). In this study, the chicken gizzard telokin cDNA and gene were cloned and analyzed. The telokin cDNA coded 157 amino acid residues which were completely identical to the C-terminal portion of the amino acid sequence of chicken gizzard MLCK. The telokin gene was coded in a 6.3-kb EcoRI genomic fragment and it consisted of three exons. The 5'-leader sequence of the telokin cDNA and genomic sequence revealed that the telokin gene was included in the MLCK gene and the transcription started in the intronic sequence of the MLCK gene. The analysis of the telokin gene suggests that the telokin expression was under the control of an independent promotor. Northern blotting and the reverse transcriptase-polymerase chain reaction methods revealed that telokin was expressed not only in chicken gizzard but also in chicken heart, lung, intestine, and skeletal muscle although the levels of the expression in the latter were much less than that in the gizzard.

The heterodimer calmodulin: myosin light-chain kinase as a prototype vertebrate calcium signal transduction complex

The heterodimer complex of calmodulin (CaM) and the protein kinase catalytic subunit of myosin light chain kinase from vertebrate smooth muscle and non-muscle tissues (sm/nmMLCK) is one of the most extensively characterized CaM-regulated enzyme complexes and it has an established in vivo role in the transduction of calcium signals into biological responses. We have used a combination of approaches to the study of CaM and sm/nmMLCK in order to derive initial insight into the key features of each protein and of the CaM-MLCK heterodimeric complex that are involved in protein-protein and calcium-protein recognition and regulation of enzyme activity. On-going studies are described here that include site-specific mutagenesis, fluorescence spectroscopy, enzymology and peptide analog analysis. These and previous results indicate that: (1), both electrostatic and hydrophobic features are important in the functionally correct interactions between CaM and MLCK; (2), even the interactions between CaM and peptide analogs of the CaM binding site of MLCK are heterogeneous and non-trivial in nature; (3), amino-acid residues that have been conserved in CaM across millions of years of evolution and that are conserved in CaMs with quantitative MLCK activator activity can be mutated without any detectable effect on activity and (4), structures different from the prototypical EF-hand domain of CaM can have similar calcium-binding activity in the presence of a CaM binding structure.

Definition of the inhibitory domain of smooth muscle myosin light chain kinase by site-directed mutagenesis

Site-directed mutagenesis of smooth muscle myosin light chain kinase was applied to define its autoinhibitory domain. Mutants were all initiated at Leu-447 but contained varying lengths of C-terminal sequence. Those containing the complete C-terminal sequence to Glu-972 possessed kinase activities that were calmodulin-dependent. Removal of the putative inhibitory domain by truncation to Thr-778 resulted in generation of a constitutively active (calmodulin-independent) species. Thus, the inhibitory domain lies to the C-terminal side of Thr-778. Truncation to Lys-793 and to Trp-800 also resulted in constitutively active mutants, although the specific activity of the latter was less than the other mutants. None of the truncated mutants bound calmodulin. For each mutant, the Km values with respect to ATP and to the 20,000-dalton light chain were similar to values obtained with the native enzyme. The presence of the inhibitory domain was detected by activation of kinase activity following limited proteolysis with trypsin. Using this procedure, it was determined that the inhibitory domain was manifest only in the mutant truncated to Trp-800 and was absent from that ending at Lys-793. These results indicate that a critical region of the inhibitory domain is contained within the sequence Tyr-794 to Trp-800. This region overlaps with the calmodulin-binding site for five residues. Our assignment of the inhibitory sequence is consistent with autoinhibition via a pseudosubstrate domain.

Regulation of smooth muscle myosin light chain kinase by calmodulin

The mutagenesis work described in this paper has been instrumental in furthering our understanding of how CaM binds to and activates MLCK. Figure 2 schematically represents this interaction. The inactive MLCK appears to have a catalytic domain that is repressed by a substrate inhibitory domain that overlaps with the CaM binding domain, a basic amphipathic helix. In the presence of Ca2+, CaM undergoes a conformational change that exposes two hydrophobic pockets, one in each globular lobe, that are important for binding to MLCK. Upon binding CaM, MLCK undergoes a conformational change that derepresses the catalytic site, allows substrate access and light chain phosphorylation. Calmodulin antagonist drugs intercalate within these hydrophobic pockets to interfere with target enzyme binding. The total loss of activity if W800 is altered to A illustrates the importance of these hydrophobic interactions within the enzyme. The basic residues are also important; most of the basic residues in the binding domain of MLCK appear to aid in CaM binding but are not in themselves crucial, this includes the RRK triad. However, a specific electrostatic interaction between R812 of MLCK and CaM is suggested by the complete failure in MLCK activation if this residue is changed to an A. Electrostatic interactions between MLCK and CaM are also indicated by the TaM-BM1 mutant. This mutant can bind to but not activate MLCK. It is hypothesized that TaM-BM1 will bind to the basic amphipathic helix of MLCK but that the alterations in the surface charges (especially E14 and T34) and/or hydrophobicity (S38) prevent the proper conformational change in MLCK necessary for light chain phosphorylation. The resulting MLCK-CaM complex is therefore, inactive but can bind TaM-BM1. The exact interaction of these amino acids in CaM with MLCK will have to await the elucidation of a CaM-MLCK co-crystal.

Regulatory functions of calmodulin

Calmodulin is a Ca2+ binding protein present in all eukaryotic cells that serves as the primary intracellular receptor for Ca2+. This 148 amino acid protein is involved in activation of more than 20 enzymes which mediate a wide variety of physiological processes. Many of these enzymes are inhibited in an intramolecular manner and the Ca(2+)-calmodulin complex relieves this inhibition. Calmodulin is essential for life as disruption of the gene in genetically tractable organisms is lethal. This protein plays important regulatory roles in cell proliferation and is required at multiple points in the cell cycle. The mechanism of enzyme activation by calmodulin and its importance in cell growth regulation are reviewed.