| Here we demonstrate that in addition to phosphorylating S6K1 and SGK 1 at their hydrophobic motif , NEK 6 also phosphorylates S6K1 at two other sites and phosphorylates SGK 1 at one other site in vitro . ^^^ Its mutation to either Ile or Val severely reduced the efficacy with which NEK 6 phosphorylated peptide substrates , and moreover , mutation of the equivalent Leu residue in S6K1 or SGK 1 prevented phosphorylation of their hydrophobic motifs by NEK 6 in vitro . ^^^ This study provides the first description of the basis for the substrate specificity of NEK 6 and indicates that NEK 6 is unlikely to be responsible for the IGF 1 induced phosphorylation of the hydrophobic motif of S6K , SGK , and protein kinase B isoforms in vivo . . ^^^ |