PRELIMINARY 3-DIMENSIONAL MODEL FOR NEMATODE THICK FILAMENT CORE

Understanding the structure and the mechanism of assembly of thick fil aments have been longstanding problems in the field of muscle biology. Cores which represent the backbones of thick filaments and consist of paramyosin and associated proteins were isolated from the nematode Ca enorhabditis elegans. Electron microscopy of negatively stained and fr ozen hydrated cores was performed, The resulting images were analyzed by computing their Fourier transforms, three-dimensional reconstructio n, and by modeling, A preliminary three-dimensional model is proposed in which the paramyosin constitutes an outer sheath of seven subfilame nts about a set of inner 54-nm-long tubules which repeat every 72 nm. The subfilaments are not closely packed but require cross-linking by t he internal tubules. Each subfilament consists of two strands of param yosin molecules which are staggered by 72 nm with respect to one anoth er. This stagger introduces a 22-nm gap between consecutive paramyosin molecules in each strand, An offset of the center of the inner tubule s relative to the center of the gap of 6 nm was consistent with the im ages and their transforms. This model suggests that the nonhelical end s of paramyosin and the unpaired gap between adjacent paramyosin molec ules contain sites for the interaction with the inner tubular proteins , The molecular interactions at this locus would appear to be critical in the assembly of thick filaments and their regulation. (C) 1995 Aca demic Press, Inc.