STEM analysis of Caenorhabditis elegans muscle thick filaments: Evidence for microdifferentiated substructures

In the thick filaments of body muscle in Caenorhabditis elegans, myosin A a nd myosin B isoforms and a subpopulation of paramyosin, a homologue of myos in heavy chain rods, are organized about a tubular core. As determined by s canning transmission electron microscopy, the thick filaments show a contin uous decrease in mass-per-length (MPL) from their central zones to their po lar regions. This is consistent with previously reported morphological stud ies and suggests that both their content and structural organization are mi crodifferentiated as a function of position. The cores are composed of a se cond distinct subpopulation of paramyosin in association with the alpha bet a, and gamma -filagenins. MPL measurements suggest that cores are formed fr om seven subfilaments containing four strands of paramyosin molecules, rath er than the two originally proposed. The periodic locations of the filegeni ns within different regions and the presence of a central zone where myosin A is located, implies that the cores are also microdifferentiated with res pect to molecular content and structure. This differentiation may result fr om a novel "induced strain" assembly mechanism based upon the interaction o f the filagenins, paramyosin and myosin A. The cores may then serve as "dif ferentiated templates" for the assembly of myosin B and paramyosin in the t apering, microdifferentiated polar regions of the thick filaments. (C) 2001 Academic Press.