Phototactic migration of Dictyostelium cells is linked to a new type of gelsolin-related protein

The molecular and functional characterization of a 125-kDa Ca2+-extractable protein of the Triton X-100-insoluble fraction of Dictyostelium cells iden tified a new type of a gelsolin-related molecule. Ln addition to its five g elsolin segments, this gelsolin-related protein of 125 kDa (GRP125) reveals a number of unique domains, two Of which are predicted to form coiled-coil regions. Another distinct attribute of GRP125 concerns the lack of sequenc e elements known to be essential for characteristic activities of gelsolin- like proteins, i.e. the severing, capping, or nucleation of actin filaments . The subcellular distribution of GRP125 to vesicular compartments suggests an activity of GRP125 different from actin-binding, gelsolin-related prote ins. GRP125 expression is tightly regulated and peaks at the transition to the multicellular pseudoplasmodial stage of Dictyostelium development. GRP1 25 was found indispensable for slug phototaxis, because slugs fail to corre ctly readjust their orientation in the absence of GRP125. Analysis of the G RP125-deficient mutant showed that GRP125 is required for coupling photodet ection to the locomotory machinery of slugs. We propose that GRP125 is esse ntial in the natural environment for the propagation of Dictyostelium spore s. We also present evidence for further representatives of the GRP125 type in Dictyostelium, as well as in heterologous cells from lower to higher euk aryotes.