Unique behavior and function of the mitochondrial ribosomal protein S4 (RPS4) in early Dictyostelium development

Certain proteins encoded by mitochondrial DNA (mt-DNA), including mt-ribosomal protein S4 (rps4), appear to play important roles in the initiation of cell differentiation. Partial disruption of rps4 in Dictyostelium discoideum Ax-2 cells by means of homologous recombination greatly impairs the progression of differentiation, while the the rps4(OE) cells in which the rps4 mRNA was overexpressed in the extra-mitochondrial cytoplasm exhibit enhanced differentiation (Inazu et al., 1999). We have prepared a specific anti-RPS4 antibody and generated transformants (rps4(AS) cells) by antisense-mediated gene inactivation of rps4. Surprisingly, in the rps4(AS) cells the progress of differentiation was found to be markedly inhibited, suggesting that the antisense rps4 RNA synthesized in the extra-mitochondrial cytoplasm might be as effective as the partial disruption of rps4 gene. Immunostaining of the rps4(OE) cells with the anti-RPS4 antibody demonstrated that the RPS4 protein synthesized in the extra-mitochondrial cytoplasm is capable of moving to the nucleus, as predicted by PSORTII. Taken together with the results obtained using immunostained Ax-2 cells, we propose a possible pathway of RPS4 translocation coupled with differentiation.