Structural studies of rabbit skeletal muscle myosin light chain kinase with monoclonal antibodies

Smooth muscle myosin light chain kinase expression in cardiac and skeletal muscle

The purpose of this study was to characterize myosin light chain kinase (MLCK) expression in cardiac and skeletal muscle. The only classic MLCK detected in cardiac tissue, purified cardiac myocytes, and in a cardiac myocyte cell line (AT1) was identical to the 130-kDa smooth muscle MLCK (smMLCK). A complex pattern of MLCK expression was observed during differentiation of skeletal muscle in which the 220-kDa-long or "nonmuscle" form of MLCK is expressed in undifferentiated myoblasts. Subsequently, during myoblast differentiation, expression of the 220-kDa MLCK declines and expression of this form is replaced by the 130-kDa smMLCK and a skeletal muscle-specific isoform, skMLCK in adult skeletal muscle. These results demonstrate that the skMLCK is the only tissue-specific MLCK, being expressed in adult skeletal muscle but not in cardiac, smooth, or nonmuscle tissues. In contrast, the 130-kDa smMLCK is ubiquitous in all adult tissues, including skeletal and cardiac muscle, demonstrating that, although the 130-kDa smMLCK is expressed at highest levels in smooth muscle tissues, it is not a smooth muscle-specific protein.

Pyruvate:NADP+ oxidoreductase from Euglena gracilis: limited proteolysis of the enzyme with trypsin

Pyruvate:NADP+ oxidoreductase from Euglena gracilis, a homodimeric protein with a molecular weight of 309 kDa, is an iron-sulfur flavoenzyme that contains thiamin pyrophosphate (TPP). The functional structure of the enzyme was studied by a limited proteolysis experiment using trypsin. The evidence obtained shows that the enzyme consists of two functional domains, one of which contains an iron-sulfur cluster, which can be isolated as a homodimeric fragment of approximately 220 kDa by proteolysis. The other domain that contains FAD is released as a monomeric fragment of approximately 55 kDa. The pyruvate dehydrogenase reaction is still catalyzed by the large fragment when NADP+ is substituted by methyl viologen, while the small fragment retains a diaphorase-like electron-transfer activity from NADPH to MV. It is thus shown that pyruvate is oxidized in a CoA-dependent reaction to form CO2 and acetyl-CoA in the iron-sulfur domain, and that the two electrons formed are transferred to the FAD domain in which NADP+ is reduced. TPP is considered to be associated in the iron-sulfur domain. The NH2-terminal sequences of the enzyme and its proteolytic fragments reveal that the iron-sulfur domain occurs in the NH2-terminal side of the enzyme. For elucidation of the O2 instability of the enzyme, limited proteolysis was attempted in air. The tryptic fragment derived from the iron-sulfur domain, similar to the native enzyme, appears to be inactivated by direct contact with O2. In contrast, the FAD domain, when separated from the other domain, is quite stable in air, although the diaphorase activity decays when the native enzyme is exposed to O2.

Molecular characterization of rat skeletal muscle myosin light chain kinase

A 1.85-kilobase (kb) cDNA has been isolated that encodes the catalytic and calmodulin binding domains of rat skeletal muscle myosin light chain kinase. The cDNA hybridized to a 3.3-kb RNA present in fast- and slow-twitch skeletal muscles. The reported enzymatic activity (3-fold greater in fast- than slow-twitch skeletal muscles) reflects the relative abundance of this RNA in the two types of skeletal muscle. No hybridization of the cDNA was detected to RNA isolated from smooth or nonmuscle tissues. The clone cross hybridized to a 2.2-kb RNA present in cardiac tissue. Ribonuclease protection analysis of skeletal and cardiac muscle RNA revealed major differences in the two hybridizing RNAs. Thus rat skeletal muscle contains a single myosin light chain kinase isoform, which is distinct from the cardiac, smooth, and nonmuscle forms.

Properties of a monoclonal antibody directed to the calmodulin-binding domain of rabbit skeletal muscle myosin light chain kinase

A synthetic peptide representing the calmodulin-binding domain of rabbit skeletal muscle myosin light chain kinase (K-R-R-W-K-K-N-F-I-A-V-S-A-A-N-R-F-K-K-I-S-S-S-G-A-L) was used as an antigen to produce a monoclonal antibody. The antibody (designated MAb RSkCBP1, of the IgM class) reacted with similar affinity (KD approximately 20 nM) by competitive enzyme-linked immunoassay (ELISA) with the antigen peptide and intact rabbit skeletal muscle myosin light chain kinase. MAb RSkCBP1 inhibited rabbit skeletal muscle myosin light chain kinase activity competitively with respect to calmodulin (Ki = 20 nM). The antibody also inhibited myosin light chain kinase activity in extracts of skeletal muscle from several mammalian species (rabbit, sheep, and bovine) and an avian species (chicken). The concentration of MAb RSKCBP1 required for 50% inhibition of enzyme activity was similar for the mammalian species (80 nM) but was significantly higher for the avian species (1.2 microM). A competitive ELISA protocol was used to analyze weak cross-reactivity to other calmodulin-binding peptides and proteins. This assay demonstrated no cross-reactivity with the venom peptides melittin or mastoparan; smooth muscle myosin light chain kinases from hog carotid, bovine trachea, or chicken gizzard; bovine brain calmodulin-dependent calcineurin; or rabbit skeletal muscle troponin I. These data support the contention that the synthetic peptide used as the antigen represents the calmodulin-binding domain of rabbit skeletal muscle myosin light chain kinase and that the calmodulin-binding domains of different calmodulin-regulated proteins may have distinct primary and/or higher order structures.