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 obliquely. This type of muscle allows force production over a wide range of lengths and is beneficial for soft-bodied animals. Unlike vertebrate cross-striated muscles, 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 α-actinin, the dogma is that dense bodies are equivalent to the Z-lines anchoring the actin barbed ends. However, we show that the barbed ends of sarcomeric actin filaments in the nematode Caenorhabditis elegans body wall muscle are aligned linearly without converging at the dense bodies. Colocalization of F-actin and ATN-1/α-actinin was negatively correlated. CAP-1, an α-subunit of capping protein/CapZ, was linearly aligned without concentration at the dense bodies independently of ATN-1. Depletion of the capping protein subunit, CAP-1 or CAP-2, caused embryonic or larval lethality with severe actin disorganization in 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.