Equatorial A-band and I-band X-ray diffraction from relaxed and active fish muscle. Further details of myosin crossbridge behaviour.

It has been known for many years that the vertebrate striated muscle A-bands and I-bands both contribute to the observed equatorial X-ray diffraction patterns. Despite this, the observed equatorial patterns, with the exception of the clearly distinct Z-reflection, have often been analysed as coming solely from the A-band, since it has not been possible to separate the observed intensity distribution into individual A-band and I-band contributions. Here we show, for the case of diffraction from the highly ordered muscles in bony fish, that it is possible to separate these contributions to the diffraction patterns from intact muscles and to compute separate electron density maps for the A and I-bands. Difference A-band density maps between resting and active muscles are distinctly altered when the I-band contribution is removed from the observed equatorial intensity. Results from resting and fully active fish muscle A-bands are compared and interpreted in terms of myosin crossbridge movement; the observations are consistent with specific crossbridge labelling of actin filaments, with a strong azimuthal component of crossbridge movement towards actin. From electron microscopy of freeze-substituted fish muscle, it is shown that the I-band X-ray diffraction pattern probably arises mainly from the thin filament arrangement immediately adjacent to the Z-band.