Plant S6 kinases do not require hydrophobic motif phosphorylation for activity in yeast lacking Ypk3

The ribosomal protein S6 kinases (S6K) are among the major substrates and crucial effectors of the target of rapamycin (TOR) kinase, which is an evolutionarily conserved regulator of cell growth and proliferation. Recent research indicates that yeast Ypk3 is an ortholog of mammalian S6Ks. Here, we find that plant S6Ks restore ribosomal protein S6 phosphorylation in a rapamycin-sensitive manner in yeast cells lacking Ypk3. However, phosphorylation of a hydrophobic motif, which is mediated through TOR signaling and essential for mammalian S6K activity, is not detected in plant S6Ks. Furthermore, deletion of the N-terminal region of rice S6Ks shows phosphorylation of the hydrophobic motif and reduced rapamycin sensitivity. Our findings suggest a mechanism of plant S6K activation distinct from that of mammalian S6Ks.