title: Novel functions of ribosomal protein S6 in growth and differentiation of Dictyostelium cells.
abstract: We have previously shown that in Dictyostelium cells a 32 kDa protein is rapidly and completely dephosphorylated in response to starvation that is essential for the initiation of differentiation (Akiyama & Maeda 1992). In the present work, this phosphoprotein was identified as a homologue (Dd-RPS6) of ribosomal protein S6 (RPS6) that is an essential member for protein synthesis. As expected, Dd-RPS6 seems to be absolutely required for cell survival, because we failed to obtain antisense-RNA mediated cells as well as Dd-rps6-null cells by homologous recombination in spite of many trials. In many kinds of cell lines, RPS6 is known to be located in the nucleus and cytosol, but Dd-RPS6 is predominantly located in the cell cortex with cytoskeletons, and in the contractile ring of just-dividing cells. In this connection, the overexpression of Dd-RPS6 greatly impairs cytokinesis during axenic shake-cultures in growth medium, resulting in the formation of multinucleate cells. Much severe impairment of cytokinesis was observed when Dd-RPS6-overexpressing cells (Dd-RPS6(OE) cells) were incubated on a living Escherichia coli lawn. The initiation of differentiation triggered by starvation was also delayed in Dd-RPS6(OE) cells. In addition, Dd-RPS6(OE) cells exhibit defective differentiation into prespore cells and spores during late development. Thus, it is likely that the proper expression of Dd-RPS6 may be of importance for the normal progression of late differentiation as well as for the initiation of differentiation.
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan.
authors.forename: Kazutaka
authors.initials: K
authors.lastname: Ishii
authors.forename: Yusaku
authors.initials: Y
authors.lastname: Nakao
authors.forename: Aiko
authors.initials: A
authors.lastname: Amagai
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Maeda
mesh_terms: D015153:Blotting, Western; D002453:Cell Cycle; D002454:Cell Differentiation; D049109:Cell Proliferation; D048749:Cytokinesis; D004023:Dictyostelium / Q000166:cytology / Q000235:genetics / Q000502:physiology*; D005455:Fluorescent Antibody Technique; D015870:Gene Expression; D010766:Phosphorylation; D015800:Protozoan Proteins / Q000235:genetics / Q000502:physiology*; D038601:Ribosomal Protein S6 / Q000235:genetics / Q000502:physiology*; D013217:Starvation
chemical_list: D015800:Protozoan Proteins; D038601:Ribosomal Protein S6