Human cardiac myosin light chains: sequence comparisons between myosin LC1 and LC2 from normal and idiopathic dilated cardiomyopathic hearts

A novel, in-solution separation of endogenous cardiac sarcomeric proteins and identification of distinct charged variants of regulatory light chain

The molecular conformation of the cardiac myosin motor is modulated by intermolecular interactions among the heavy chain, the light chains, myosin binding protein-C, and titin and is governed by post-translational modifications (PTMs). In-gel digestion followed by LC/MS/MS has classically been applied to identify cardiac sarcomeric PTMs; however, this approach is limited by protein size, pI, and difficulties in peptide extraction. We report a solution-based work flow for global separation of endogenous cardiac sarcomeric proteins with a focus on the regulatory light chain (RLC) in which specific sites of phosphorylation have been unclear. Subcellular fractionation followed by OFFGEL electrophoresis resulted in isolation of endogenous charge variants of sarcomeric proteins, including regulatory and essential light chains, myosin heavy chain, and myosin-binding protein-C of the thick filament. Further purification of RLC using reverse-phase HPLC separation and UV detection enriched for RLC PTMs at the intact protein level and provided a stoichiometric and quantitative assessment of endogenous RLC charge variants. Digestion and subsequent LC/MS/MS unequivocally identified that the endogenous charge variants of cardiac RLC focused in unique OFFGEL electrophoresis fractions were unphosphorylated (78.8%), singly phosphorylated (18.1%), and doubly phosphorylated (3.1%) RLC. The novel aspects of this study are that 1) milligram amounts of endogenous cardiac sarcomeric subproteome were focused with resolution comparable with two-dimensional electrophoresis, 2) separation and quantification of post-translationally modified variants were achieved at the intact protein level, 3) separation of intact high molecular weight thick filament proteins was achieved in solution, and 4) endogenous charge variants of RLC were separated; a novel doubly phosphorylated form was identified in mouse, and singly phosphorylated, singly deamidated, and deamidated/phosphorylated forms were identified and quantified in human non-failing and failing heart samples, thus demonstrating the clinical utility of the method.

Calcium regulation in the human myocardium affected by dilated cardiomyopathy: a structural basis for impaired Ca2+-sensitivity

Calcium regulation in the human heart is impaired during idiopathic dilated cardiomyopathy (IDC). Here, we analyze the structural basis for impairment in the regulatory mechanism. Regulation of contractility was monitored by MgATPase and Ca2+-binding assays as a function of calcium. Myofibrillar proteolysis and expression of troponin T isoforms were established by gel electrophoresis and by Western blots. Myofibrillar ATPase assays in low salt however, revealed a drastic lowering of calcium sensitivity in IDC myofibrils as indicated by reductions in both activation by high calcium and in EGTA-mediated inhibition of MgATPase. Structural changes in myofilament proteins were found in most IDC hearts, specifically proteolysis of myosin light chain 2 (LC2), troponin T and I (TnT and TnI), and sometimes a large isoform shift in TnT. IDC did not induce mutations in LC2 and troponin C (TnC), as established by cDNA sequence data from IDC cases, thus, calcium binding to IDC myofibrils was unaffected. Reassociation of IDC myofibrils with native LC2 raised MgATPase activation at high Ca2+ to control levels, while repletion with intact, canine TnI/TnT restored inhibition at low Ca2+. A model, identifying possible steps in the steric blocking mechanism of regulation, is proposed to explain IDC-induced changes in Ca2+-regulation. Moreover, shifts in TnT isoforms may imply either a genetic or a compensatory factor in the development and pathogenesis of some forms of IDC.

Cloning of the cDNA and nucleotide sequence of a skeletal muscle protease from myopathic hamsters

A neutral protease with an estimated Mr of about 26 kD and responsible for cleavage ofmyosin LC2 was isolated from hamster skeletal muscle. Complementary DNAs were generated by RT-PCR using total hamster muscle RNA and degenerate oligonucleotide primers based on the sequences of two internal peptides. The nucleotide sequences of the resultant cDNAs were subsequently determined and the complete amino acid sequence of the protease deduced. Although the hamster protein shared 63-85% identity in nucleotide and amino acid sequences with rat and mouse mast cell proteases, it had a higher degree of specificity for myosin LC2 than mast cell proteases which also digested myosin LC1 and myosin heavy chains. As a result, the hamster protease was designated mekratin because of its unique enzymatic specificities to distinguish it from other mast cell proteases. A polyclonal antibody was raised specific to the hamster muscle and human cardiac muscle mekratins without apparent cross-reaction with rat mast cell proteases. We have earlier demonstrated the presence in excess of a neutral protease that specifically cleaves LC2 in human hearts obtained at end stage idiopathic dilated cardiomyopathy (IDC). Western analyses revealed that heart tissue from patients with IDC contained 5-10 fold more mekratin than control samples. Furthermore, the level of the protease in human IDC tissues was similar to that seen in myopathic hamster skeletal muscle. No bands were recognized by the antibody when IDC myofibrils were probed due to the removal of soluble proteins during sample preparation. Thus, these results strongly suggest that the anti-mekratin antibody will provide positive identification of IDC in many cases and diagnosis by exclusion may be replaced.