Linear Z-line-like alignment of capping protein in obliquely striated muscle of the nematode <i>C. elegans</i> 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 the 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 <i>Caenorhabditis elegans</i> 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 <i>C. elegans</i> muscle and suggest a new model of a linear Z-line-like arrangement of actin barbed ends.