Proteomic Analysis of Importin alpha-interacting Proteins in Adult Mouse Brain

Many transport factors, such as importins and exportins, have been identified, and the molecular mechanisms underlying nucleocytoplasmic transport have been characterized. The specific molecules that are carried by each transport factor and the temporal profiles that characterize the movements of various proteins into or out of the nucleus, however, have yet to be elucidated. Here, we used a proteomic approach to identify molecules that are transported into the nuclei of adult mouse brain cells via importin alpha 5. We identified 48 proteins in total, among which we chose seven to characterize more extensively: acidic (leucine-rich) nuclear phosphoprotein 32 family member A (Anp32a), far upstream element binding protein 1 (FUBP1), thyroid hormone receptor beta 1 (TR beta 1), transaldolase 1, CDC42 effector protein 4 (CDC42-ep4), Coronin 1B, and brain-specific creatine kinase (CK-B). Analyses using green fluorescent protein (GFP)-fused proteins showed that Anp32a, FUBP1, and TR beta 1 were localized in the nucleus, whereas transaldolase 1, CDC42-ep4, CK-B, and Coronin 1B were distributed in both the cytoplasm and nucleus. Using a digitonin-permeabilized in vitro transport assay, we demonstrated that, with the exception of CK-B, these proteins were transported into the nucleus by importin alpha 5 together with importin beta and Ran. Further, we found that leptomycin B (LMB) treatment increased nuclear CK-B-GFP signals, suggesting that CK-B enters the nucleus and is then exported in a CRM1-dependent manner. Thus, we identified a comprehensive set of candidate proteins that are transported into the nucleus in a manner dependent on importin alpha 5, which enhances our understanding of nucleocytoplasmic signaling in neural cells.