Publication Date:
2013-07-30
Description:
The activity of S6K1 (p70 ribosomal protein subunit 6 kinase 1) is stimulated by phosphorylation of Thr 389 in the hydrophobic motif by mTORC1 (mammalian target of rapamycin complex 1) and phosphorylation of Thr 229 in the activation loop by PDK1 (phosphoinositide-dependent kinase 1); however, the order of the two events is still ambiguous. In the present paper we report six crystal structures of the S6K1 kinase domain alone or plus the hydrophobic motif in various forms, in complexes with a highly specific inhibitor. The structural data, together with the biochemical data, reveal in vivo phosphorylation of Thr 389 in the absence of Thr 229 phosphorylation and demonstrate the importance of two conserved residues, Gln 140 and Arg 121 , in the establishment of a hydrogen-bonding network between the N-lobe (N-terminal lobe) and the hydrophobic motif. Phosphorylation of Thr 389 or introduction of a corresponding negatively charged group leads to reinforcement of the network and stabilization of helix αC. Furthermore, comparisons of S6K1 with other AGC (protein kinase A/protein kinase G/protein kinase C) family kinases suggest that the structural and sequence differences in the hydrophobic motif and helix αC account for their divergence in PDK1 dependency. Taken together, the results of the present study indicate that phosphorylation of the hydrophobic motif in S6K1 is independent of, and probably precedes and promotes, phosphorylation of the activation loop.
Print ISSN:
0264-6021
Electronic ISSN:
0006-2936
Topics:
Biology
,
Chemistry and Pharmacology
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