Electron microscopy and x-ray diffraction evidence for two Z-band structural states

Area of Science:

• Muscle physiology
• Biophysics
• Cell biology

Background:

• Z-bands link sarcomeres in vertebrate muscles and contain signaling proteins.
• Z-bands may function as strain sensors.
• Previous electron microscopy revealed two Z-band structures: relaxed 'small-square' and active 'basketweave', but the transition mechanism was unknown.

Purpose of the Study:

• Investigate the physical factors influencing Z-band structure.
• Determine the mechanism underlying the Z-band structural transition during muscle activation.

Main Methods:

• Electron microscopy (EM) of rabbit psoas muscle under varying temperature, osmotic pressure, and ionic strength.
• X-ray diffraction of unfixed muscles to detect Z-band structures in situ.
• Ca(2+) application with force inhibitors to probe activation mechanisms.

Main Results:

• The ratio of small-square to basketweave Z-band structures varied with temperature and osmotic pressure, but not ionic strength, in relaxed muscle.
• EM showed Z-band lattice spacing varied with temperature and pressure, with basketweave spacing 10% larger than small-square.
• X-ray diffraction revealed two Z-reflections consistent with EM findings after accounting for processing shrinkage.
• Z-band form interconversion correlated with tropomyosin movement on actin.

Conclusions:

• The two Z-band forms observed by EM correspond to distinct X-ray diffraction reflections.
• The transition between Z-band structures during muscle activation is linked to actin-tropomyosin dynamics.