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14-3-3 proteins are phosphoserine/threonine-binding proteins that play important roles in many regulatory processes, including intracellular protein targeting. 14-3-3 proteins can anchor target proteins in the cytoplasm and in the nucleus or can mediate their nuclear export. So far, no role for 14-3-3 in mediating nuclear import has been described. There is also mounting evidence that nuclear import is regulated by the phosphorylotion of cargo proteins, but the underlying mechanism remains elusive. Myopodin is a dual-comportment, actin-bundling protein that functions as a tumor suppressor in human bladder cancer. In muscle cells, myopodin redistributes between the nucleus and the cytoplasm in a differentiation-dependent and stress-induced fashion. We show that importin a binding and the subsequent nuclear import of myopodin are regulated by the serine/threonine phosphorylation-dependent binding of myopodin to 14-3-30 These results establish a novel paradigm for the promotion of nuclear import by 14-3-3 binding. They provide a molecular explanation for the phosphorylation-dependent nuclear import of nuclear localization signal-containing cargo proteins. [ABSTRACT FROM AUTHOR]