Arabidopsis S6 Kinase, a cell growth regulator involved in repression of cell proliferation

Seminar

Title: Arabidopsis S6 Kinase, a cell growth regulator involved in repression of cell proliferation

Speaker: Rossana Henriques

Affiliation: Laboratory of Plant Molecular Biology, The Rockefeller University, New York, NY, USA
 

Abstract

The 40S ribosomal S6 Kinase belongs to the AGC family of protein kinases and is involved in the phosphorylation of the ribosomal protein S6. In yeast, animals and plants, S6 kinase was identified as a conserved component of signaling pathways controlling cell growth. Arabidopsis S6 Kinases (S6K1 and S6K2) share similarities with their animal counterparts but possess a different structure, with a specific N-terminal domain, of unknown function.
Although depletion of S6Ks is lethal, hemizygous s6k1s6k2/++ mutants and S6K1 RNAi lines show high phenotypic instability with variation in size, and increased trichome branching, produce non-viable pollen and high levels of aborted seeds. Analysis of their DNA content by flow cytometry, as well as chromosome counting using DAPI staining and FISH, revealed an increase in ploidy due to elevated chromosome number. In agreement with this data, S6K1 was found to associate with the RBR1-E2FB complex via its plant specific N-terminal LVxCxE motif. Moreover, the S6K1-RBR1 association regulates RBR1 nuclear localization, as well as E2F-dependent expression of cell cycle genes. Arabidopsis cells grown under nutrient-limiting conditions require S6K for repression of cell proliferation. These results suggest that S6Ks, besides their role in growth control, could also act as repressors of cell proliferation that are required for maintenance of chromosome stability and ploidy levels.
A future challenge would be to understand how S6K can integrate growth regulation and cell proliferation. Interestingly, it has been shown that S6K1 is co-expressed with CCA1, a master regulator of the Arabidopsis circadian clock. Furthermore, the fact that S6K1 message accumulates in the end of the night (when the elongation growth is maximal), suggests the possibility that S6K1 could be involved in a mechanism to regulate growth under the control of the circadian clock.
Further understanding of how S6K transcription, protein and kinase activity levels vary during the day could provide important tools for the understanding of growth/cell division control in a physiological set up.