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Cyanogen bromide fragments of the cardiac I light chain from bovine myosin: Evidence for sequence homology with rabbit skeletal myosin alkali light chains
Author(s) -
Weeds Alan G.
Publication year - 1975
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/0014-5793(75)80375-9
Subject(s) - cyanogen bromide , myosin , immunoglobulin light chain , chemistry , microbiology and biotechnology , biology , biochemistry , peptide sequence , genetics , antibody , gene
It has recently been shown that there is extensive homology of amino acid sequence between rabbit skeletal muscle myosin alkali light chains, rabbit troponin C and a calcium binding parvalbumin from carp [l-3] , which indicates that these proteins may have evolved from a common precursor. Ititernal repeats within all three sequences suggest that gene duplication has occurred during their evolution. The structure of carp parvalbumin, which contains two calcium binding sites, has been determined by X-ray analysis [4]. Each site is located in a corner between two helical regions, giving a basic structural unit of the type: Helix A-Site S-Helix B (A-S-B). The calcium ions are ligated to a number of acidic residues within each site [4]. Duplication of this unit produces the two-site structure Al-Sl-Bl-L-A2-S2-B2, where L is a link region. The parvalbumin contains two additional helices at its N-terminal end, and the six helices pack together to form a hydrophobic core which stabilises the three-dimensional structure. The existence of this core depends on certain apolar residues located at specific positions within each helix forming van der Waals’ interactions between the different helices. Similar groupings have been observed in the apolar residues of both troponin C and the alkali light chains, which suggest that these proteins may contain similar structural elements. Troponin C binds four calcium ions [S] , and if the sequence is drawn out to correspond to the structural features of the parvalbumin, four units of the A-S-B type are produced 123. Furthermore, within each site so generated acidic residues are found corresponding to the calcium ligating positions of the parvalbumin. The sequence of the alkali light chains has also been drawn out to correspond to the same structural features [2], but in this case the ‘site’ regions do not contain the requisite numbers of acidic residues found in the other two proteins, and there is no evidence that isolated alkali light chains bind calcium. Nevertheless the preservation of the pattern of apolar residues together with the extensive sequence homology, suggest strong structural similarities between these proteins. Table 1 shows this sequence drawn out to display these structural features. One of the light chains from beef cardiac myosin contains an identical thiol sequence to that of the alkali light chains [6] but it has two other thiol groups which are absent from the rabbit protein. Cyanogen bromide fragments have been isolated from the cardiac light chain to investigate the location of these thiol groups within the sequence, and tryptic peptides from these fragments analysed. Although a complete sequence has not been produced, these preliminary studies provide evidence for extensive sequence homology between the cardiac and skeletal light chains. Furthermore, it is possible to order many of the tryptic peptides on the basis of these homologies, and the same specific patterns of apolar residues are observed within the sequence so generated.