THE RADIUS OF GYRATION OF NATIVE AND REDUCTIVELY METHYLATED MYOSIN SUBFRAGMENT-1 FROM NEUTRON-SCATTERING

Reductive methylation of nearly all lysine groups of myosin subfragmen t-1 (S1) was required for crystallization and solution of its structur e at atomic resolution. Possible effects of such methylation on the ra dius of gyration of chicken skeletal muscle myosin S1 have been invest igated by using smalt-angle neutron scattering. In addition, we have i nvestigated the effect of MgADP . V-i, which is thought to produce an analog of the S1 . ADP . P-i state, on the S1 radius of gyration. We f ind that although methylation of S1, with or without SO42- ion additio n, does not significantly alter the structure, addition of ADP plus va nadate does decrease the radius of gyration significantly. The S1 crys tal structure predicts a radius of gyration close to that measured her e by neutron scattering. These results suggest that the overall shape found by crystallography resembles nucleotide-free S1 in solution. In order to estimate the effect of residues missing from the crystal stru cture, the structure of missing loops was estimated by secondary-struc ture prediction methods. Calculations using the complete crystal struc ture show that a simple closure of the nucleotide cleft by a rigid-bod y torsional rotation of residues (172-180 to 670) around an axis runni ng along the base of the cleft alone does not produce changes as large as seen here and in x-ray scattering results. On the other hand, a ri gid body rotation of either the light-chain binding domain (767 to 843 plus light chains) or of a portion of 20-kDa peptide plus this domain (706 to 843 plus light chains) is more readily capable of producing s uch changes.