Structure of insect fibrillar flight muscle in the presence and absence of ATP

Abstract

Low-angle X-ray diffraction pictures were taken of Lethocerus flight muscle in one or other of its two inactive states, relaxation and rigor. They showed more detail than previous pictures and the parameters of the actin and myosin helices could be deduced from the spacings of the observed layer lines. Subunits consisting of actin monomers and of myosin heads were arranged on helices so as to conform to these parameters, and the diffraction patterns from these models were calculated. When these model systems resembled the structures observed in the electron microscope, the calculated diffraction patterns had layer Unes similar in radial distribution and relative intensity to those observed, with certain exceptions. The fit was quite critical, in that variation of the size and orientation of the subunits affected it considerably. The results support the idea that in rigor (i.e. without ATP) all of the myosin heads attach to actin monomers in a regular angled configuration. In contrast, on relaxation (i.e. on addition of ATP but no Ca 2+) most or all of the myosin heads detach from the actin and are arranged with a specific symmetry around the myosin filament. It is not necessary, however, to assume that they change shape or move far from the actin filament in this process. Features of the X-ray diffraction pattern which remained unaccounted for by this model can be explained on the basis of an arrangement of actin helices on a further helix around a thick filament. Types of extended lattices containing actin helices in statistical axial or azimuthal positions are discussed.