STRAD alpha regulates LKB1 localization by blocking access to importin-alpha, and by association with Crm1 and exportin-7

LKB1, a serine/threonine kinase, regulates cell polarity, metabolism, and cell growth. The activity and cellular distribution of LKB1 are determined by cofactors, STRAD alpha and MO25. STRAD alpha induces relocalization of LKB1 from the nucleus to the cytoplasm and stimulates its catalytic activity. MO25 stabilizes the STRAD alpha/LKB1 interaction. We investigated the mechanism of nucleocytoplasmic transport of LKB1 in response to its cofactors. Although LKB1 is imported into the nucleus by importin-alpha/beta, STRAD alpha and MO25 passively diffuse between the nucleus and the cytoplasm. STRAD alpha induces nucleocytoplasmic shuttling of LKB1. STRAD alpha facilitates nuclear export of LKB1 by serving as an adaptor between LKB1 and exportins CRM1 and exportin7. STRAD alpha inhibits import of LKB1 by competing with importin-alpha for binding to LKB1. MO25 stabilizes the LKB1-STRAD alpha complex but it does not facilitate its nucleocytoplasmic shuttling. Strikingly, the STRAD alpha, isoform which differs from STRAD beta in the N- and C-terminal domains that are responsible for interaction with export receptors, does not efficiently relocalize LKB1 from the nucleus to the cytoplasm. These results identify a multifactored mechanism to control LKB1 localization, and they suggest that the STRAD beta-LKB1 complex might possess unique functions in the nucleus.