The relaxed crossbridge pattern in isolated rabbit psoas muscle thick filaments

Rabbit muscle is a major source of material for biochemical experiments and spin labelling studies of contraction, and so it is important to establish how closely this material resembles the frog and fish muscles usually used for structural studies. Previous studies have shown that relaxed rabbit muscle thick filaments lose the characteristic order of their crossbridges when they are cooled below about 15-19 degrees C, whereas the order of fish and frog muscles is retained above 0 degrees C. The lack of order has frustrated attempts to examine rabbit thick filament structure and has raised questions about how closely they might resemble other thick filaments. We have therefore developed a procedure for preserving the crossbridge order in isolated filaments. Electron microscopy of these thick filaments after either negative staining or metal shadowing has shown that the crossbridge pattern has a 43 nm axial repeat and is based on three near-helical strands. Computed transforms of either type of image show a series of layer lines confirming that the native relaxed pattern has been preserved, and computer reconstructions show the individual crossbridges lying on a slightly perturbed 3-stranded lattice. These data indicate an unexpectedly high degree of similarity between the rabbit and frog patterns and indicate that, in fully preserved material, there is little structural difference between the two thick filaments at the temperature at which each normally functions.(ABSTRACT TRUNCATED AT 250 WORDS)