Figure 5. Role of cofilin in notochord cell elongation.

(A) Structure of Ciona cofilin (top) and design of cofilinS5E mutant (bottom). ADF, actin-depolymerizing factor domain. (B) Phenotype of cofilinS5E-mCherry–expressing notochord cell (median section) at 18 hpf. Two cells flanking the cofilinS5E-mCherry–expressing cell, which is indicated by a white arrowhead, are outlined by dashed lines. They are able to elongate, and exert pushing forces along the longitudinal axis, squeezing the cofilinS5E cell to assume a dumbbell shape. Actin accumulates abnormally off the equator in the cofilinS5E cell (yellow arrowheads), instead of at the equatorial region seen in a cell not expressing the mutant (yellow arrows). (C) A model suggesting that cofilinS5E disrupts the turnover of actin filaments at the equator and the flux of actin network, resulting in an elongation failure. Scissors indicate the removal, by cofilin, of actin filaments (green lines), which move toward the equator. Brown arrows indicate the longitudinal pushing forces exerted by each notochord cell at its neighbors. Scale bars, 5 µm.