title: Tropomyosin fragments obtained by tryptic digestion.
abstract: Rabbit skeletal alpha-tropomyosin, separated by hydroxyapatite chromatography, was treated with trypsin (1/100 wt/wt) at 0 degrees C for 24 h. Trypsin-resistant fragments of tropomyosin were separated into the precipitate and supernatant fractions at pH 4.3 in 1 M KCl, and these were subjected to QAE-Sephadex A50 column chromatography for further purification. SDS-gel electrophoresis showed 16,000 and 14,000 dalton bands for the supernatant (s-fragment) and an 11,500 dalton band for the precipitate (p-fragment). We obtained a 13,500 dalton chain (13,500 dalton fragment) in addition to the s- and p-fragments upon treatment with more dilute trypsin (1/500 wt/wt) for 48 h at 0 degrees C. Both the p- and 13,500 dalton fragment had the same C-terminal portion as intact alpha-tropomyosin, and could form an intra-chain disulfide bond on oxidation. Therefore, these two fragments were deduced to be polypeptides from some points on the N-terminal side of Cys 190 to the intact C-terminal. The s-fragment, on the other hand, did not contain any cysteine, Phe, or His residues according to amino acid analysis, suggesting that the fragment is derived from the N-terminal side from Cys 190. Tentative assignment of the fragments was carried out by amino acid analysis, and C- and N-terminal determination. The p-, s-, and 13,500 dalton fragments appear to be in coiled-coil form in solution, having alpha-helical contents of 77,71, and 64%, respectively, and are able to interact with intact tropomyosin to reduce the viscosity of tropomyosin solution. The s-, p-, and 13,500 dalton fragments have little binding capacity individually to troponin, but the mixture, i.e., the s- and p-fragments, the 13,500 dalton fragment and the N-chain, which was obtained by cleavage at Cys 190, showed clear binding with troponin independent of Ca2+ in solution as detected by gel electrophoresis. The p-fragment showed some binding to troponin, since cross-linkage to troponin was possible by treatment with dimethyl suberimidate. From the result, it can be inferred that the troponin binding regions in tropomyosin are located on both sides of Cys 190, where trypsin attacks more easily than at other parts of the molecule, leaving two trypsin-resistant fragments.
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D001665:Binding Sites; D055598:Chemical Phenomena; D002621:Chemistry; D008970:Molecular Weight; D010446:Peptide Fragments / Q000302:isolation & purification; D011487:Protein Conformation; D011817:Rabbits; D014335:Tropomyosin / Q000378:metabolism; D014336:Troponin / Q000378:metabolism; D014357:Trypsin
chemical_list: D000596:Amino Acids; D010446:Peptide Fragments; D014335:Tropomyosin; D014336:Troponin; D014357:Trypsin