The tumor suppressor protein kinase LKB1 exerts its effects by phosphorylating and activating AMP-activated protein kinase (AMPK) and members of the AMPK-related kinase family, such as the brain-specific kinases BRSK1/BRSK2 (SAD-B/SAD-A). LKB1 contains a conserved serine residue near the C terminus (Ser-431 in mouse LKB1) that is phosphorylated by cyclic AMP-dependent protein kinase and p90-RSK. Although some studies suggest that LKB1 is constitutively active and is not rate-limiting for activation of AMPK, others have suggested that phosphorylation of Ser-431 is necessary to allow LKB1 to phosphorylate and activate AMPK and other downstream kinases. Prompted by our discovery of an LKB1 splice variant (LKB1(S)) that lacks Ser-431, we have reinvestigated this question. In HeLa cells (which lack endogenous LKB1), co-expression with STRAD alpha and MO25 alpha of wild type LKB1, the S431A or S431E mutants of LKB1, or LKB1S gave equal levels of activation of endogenous AMPK. Similarly, recombinant STRAD alpha center dot MO25 alpha complexes containing these LKB1 variants were equally effective at phosphorylating and activating AMPK, BRSK1, and BRSK2 in cell-free assays. Finally, all four LKB1 variants and a truncated LKB1 lacking the C-terminal region altogether were equally effective at causing cell cycle arrest when co-expressed with STRAD alpha and MO25 alpha in the G361 melanoma cell line. Our results do not support the idea that phosphorylation of Ser-431 increases the ability of LKB1 to phosphorylate downstream targets.
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