C-terminal Elements Control Location, Activation Threshold, and p38 Docking of Ribosomal S6 Kinase B (RSKB)

RSKB, a p90 ribosomal S6 protein kinase with two catalytic domains, is activated by p38- and extracellular signal-regulated kinase mitogen-activated protein kinase pathways. The sequences between the two catalytic domains and of the C-terminal extension contain elements that control RSKB activity. The C-terminal extension of RSKB presents a putative bipartite713KRX14KRRKQKLRS737nuclear location signal. The distinct cytoplasmic and nuclear locations of various C-terminal truncation mutants supported the hypothesis that the nuclear location signal was essential to direct RSKB to the nuclear compartment. The725APLAKRRKQKLRS737 sequence also was essential for the intermolecular association of RSKB with p38. The activation of RSKB through p38 could be dissociated from p38 docking, because RSKB truncated at Ser681 strongly responded to p38 pathway activity. Interestingly, Δ725–772-RSKB was nearly nonresponsive to p38. Sequence alignment with the autoinhibitory C-terminal extension of Ca+2/calmodulin-dependent protein kinase I predicted a conserved regulatory 708AFN710motif. Alanine mutation of the key Phe709 residue resulted in strongly elevated basal level RSKB activity. A regulatory role also was assigned to Thr687, which is located in a mitogen-activated protein kinase phosphorylation consensus site. These findings support that the RSKB C-terminal extension contains elements that control activation threshold, subcellular location, and p38 docking.