Vertebrate muscle Z-line structure: an electron microscopic study of negatively-stained myofibrils

Structural features of the Z-lines of rabbit psoas muscle myofibrils have been studied in the electron microscope with a negative staining technique. The results obtained suggest the presence of about 20 nm periodicity in the structural organization of the Z-line region: a band pattern of five bands of extra density spaced about 20 nm apart was revealed in the Z-region and the Z-filaments connecting actin filaments from neighbouring sarcomeres often appeared to be positioned at intervals of 17-20 nm. An electron microscopic investigation of the interaction in vitro of two major Z-line proteins, alpha-actinin and F-actin, indicated that the positions of alpha-actinin bridges between actin filaments are defined by relative azimuthal positions of actin subunits. A possible arrangement of actin-linking macromolecular bridges in the Z-region is considered. It is supposed that the arrangement of the Z-filaments is related to the helical symmetry of actin-containing filaments. Also, the banded appearance of the Z-region is interpreted as arising from the arrangement of crossbridges connecting thin filaments of the same sarcomeres.