Differential assembly of alpha- and gamma-filagenins into thick filaments in Caenorhabditis elegans

Muscle thick filaments are highly organized supramolecular assemblies of my osin and associated proteins with lengths, diameters and flexural rigiditie s characteristic of their source. The cores of body wall muscle thick filam ents of the nematode Caenorhabditis elegans are tubular structures of param yosin sub-filaments coupled by filagenins and have been proposed to serve a s templates for the assembly of native thick filaments. We have characteriz ed alpha- and gamma -filagenins, two novel proteins of the cores with calcu lated molecular masses of 30,043 and 19,601 and isoelectric points of 10.52 and 11.49, respectively. Western blot and immunoelectron microscopy using affinity-purified antibodies confirmed that the two proteins are core compo nents. Immunoelectron microscopy of the cores revealed that they assemble w ith different periodicities. Immunofluorescence microscopy showed that alph a -filagenin is localized in the medial regions of the A-bands of body wall muscle cells whereas gamma -filagenin is localized in the flanking regions , and that alpha -filagenin is expressed in 1.5-twofold embryos while gamma -filagenin becomes detectable only in late vermiform embryos. The expressi on of both proteins continues throughout later stages of development. C. el egans body wall muscle thick filaments of these developmental stages have d istinct lengths. Our results suggest that the differential assembly of alph a- and gamma -filagenins into thick filaments of distinct lengths may be de velopmentally regulated.