Immunoelectron microscopic observations on tropomyosin localization in striated muscle

Tropomyosin localization in striated muscle was studied by means of immunoelectron microscopy. Polyclonal and monoclonal antibodies to tropomyosin were allowed to diffuse into mechanically skinned single fibres dissected from frog semitendinosus muscle. Antibodies produced transverse I-band stripes with the expected periodicity of 38 nm. However, some differences were revealed among the various antibodies. While polyclonal antibodies generally showed 23 stripes, monoclonal antibodies showed an extra 24th stripe immediately adjacent to the Z-line, implying some structural/functional uniqueness of this terminal tropomyosin. Furthermore, the stripes did not always lie parallel to the Z-line. When the Z-line was straight or slightly skewed, the stripes generally were parallel to it. However, when Z-line skew was more severe, the stripes remained perpendicular to the fibre axis, indifferent to the Z-line skew. This may implay that the coupling of tropomyosin to the thin filament is not tight. Finally, the monoclonal antibodies themselves exerted an anomalous effect on the Z-line, apparently extracting or shifting some of its mass.