We have cxprcsscd in E. ecUi segments of the rod portion of rabbit skeletal fart muscle myoxin and conrpnrcd physical propertied of two din'crcnt nprcics, LMM-30 and LMM-SOC', LMM-30 consirrs of 263 amino widr including the original C-tcnninus of nryosin heavy rhnin. LMM-30C is colincar with LMM-30, but is devoid of 17 rckiducs a~ the C-terminus. 'W NMR tpcctroscopy indicates that the Gtcmrinus af LMM-30, but not of LMM30C' is unfolded and freely mobile, Furthermore, the prcrcnt results show that the unfolded C+xminus is esscntinl for molecular aarrmbly of LMM-30; ut pi*1 8.0 LMM-30, but not LMM-JOC', formed agyrcyatcs upon dccrcusing the ionic rtrcngth,

Intact rabbit myosin and two different C-terminal fragments of rabbit muscle light meromyosin (LMM) expressed in Escherichia coli, LMM-30, and LMM-30C', were studied by 1H NMR spectroscopy. X-ray small-angle scattering shows that at high ionic strength two polypeptide chains of LMM-30 (which consists of the C-terminal 262 amino acids of myosin heavy chain) or LMM-30C' (which corresponds to LMM-30 but lacks the last 17 residues) assemble to form an alpha-helical coiled-coil as it is found also in myosin. The last 12 C-terminal residues of one polypeptide chain of LMM-30 and the last 9 C-terminal residues of the other chain are very mobile. The last 8 residues of the two strands are equivalent from the NMR point of view and unfolded; the valine residues in position 255 in the two strands are not equivalent, suggesting an interaction between the two strands, Ser-252, Arg-253, and Asp-254 are completely immobilized in one of the polypeptide strands and partly mobile in the other. Essentially the same pattern is observed in intact myosin. In spite of the large molecular weights of LMM-30 and LMM-30C', it is possible to resolve almost all aromatic residues and to determine the pK values of all the 4 tyrosine and of 9 (out of 10) histidine residues. The tyrosine residues in the two strands are equivalent in the two polypeptide chains and both have a pK of 10.5. The pK values of the histidine residues vary between 5.7 and 7.0.

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