Linear Z-line-like alignment of capping protein in obliquely striated muscle of the nematode C. elegans suggests that dense bodies are not equivalent to Z-lines

Many invertebrates have obliquely striated muscles, in which neighboring thin and thick filaments are staggered and aligned in an oblique manner. This type of muscle allows force production over a wide range of lengths and is beneficial for soft-bodied animals. Unlike cross-striated muscles of vertebrates, most of obliquely striated muscles lack distinct Z-lines and, instead, have dense bodies. Because the dense bodies are located in the middle of the I-bands and contain -actinin, the dogma is that dense bodies are equivalent to the Z-lines anchoring the actin barbed ends. However, we present evidence that the barbed ends of sarcomeric actin filaments in the nematode Caenorhabditis elegans body wall muscle are aligned in a linear Z-line-like arrangement without converging at the dense bodies. Colocalization of F-actin and ATN-1/-actinin was minimal. Furthermore, CAP-1, an -subunit of capping protein/CapZ, was linearly aligned in the middle of the I-bands without concentration at the dense bodies. This linear CAP-1 alignment was maintained in the absence of ATN-1. These results demonstrate that the actin barbed ends are not directly anchored to the dense bodies. Depletion of the capping protein subunit, CAP-1 or CAP-2, caused embryonic or larval lethality with severe actin disorganization in the body wall muscle, indicating that barbed-end regulation by capping protein is essential for sarcomere assembly. These results contradict the current view of the sarcomere organization in C. elegans muscle and suggest a new model of a linear Z-line-like arrangement of actin barbed ends. Significance StatementO_LIWithout clear evidence, there has been a notion that actin filaments are directly anchored to the dense bodies in C. elegans striated muscle. C_LIO_LICapping protein localizes in a linear Z-line-like alignment in C. elegans muscle without concentrating at the dense bodies, indicating that the actin barbed ends are not directly anchored at the dense bodies. C_LIO_LIDepletion of capping protein causes severe sarcomere defects in embryos and larvae indicating a critical role of capping protein in sarcomere assembly. C_LI