AtSGP1, AtSGP2 and MAP4K alpha are nucleolar plant proteins that can complement fission yeast mutants lacking a functional SIN pathway

In the fission yeast Schizosaccharomyces pombe, the onset of septum formation is signalled via the septation initiation network (SIN) involving several protein kinases and a GTPase. Arabidopsis thaliana and Brassica napus proteins homologous to fission yeast spg1p (AtSGP1, AtSGP2), cdc7p (AtMAP3K epsilon 1, AtMAP3K epsilon 2, BnMAP3K epsilon 1) and sid1p (AtMAP4K alpha 1, AtMAP4K alpha 2, BnMAP4K alpha 2) exhibit a significant similarity. The plant proteins AtSGP1/2 and BnMAP4K alpha 2 are able to complement the S. pombe mutant proteins spg1-B8 and sid1-239, respectively and to induce mutisepta when overexpressed in wild-type yeast. Yeast two-hybrid assays demonstrated interactions both between plant proteins and between plant and yeast proteins of the SIN pathway. However, the primary structure of the proteins as well as the partial complementation of yeast mutants indicates that plant homologous proteins and their yeast counterparts have diverged during evolution. Real-time RT-PCR studies demonstrated plant SIN-related gene expression in all organs tested and a co-expression pattern during the cell cycle, with a higher accumulation at G(2)-M. During interphase, the plant SIN-related proteins were found to co-localise predominantly in the nucleolus of the plant cells, as shown by fusions to green fluorescent protein. These data suggest the existence of a plant SIN-related pathway.