A New Monoclonal Antibody Enables BAR Analysis of Subcellular Importin β1 Interactomes

Changes in importin levels promote nuclear proteasomal degradation of cell cycle-related proteins during THP-1 monocyte-to-macrophage differentiation

Importin family nucleocytoplasmic transport receptors share thousands of cargo proteins. To elucidate cell regulatory mechanisms via transport regulation, we analyzed the levels of transport receptors by western blotting and quantified the total cellular and nuclear proteins during monocyte-to-macrophage differentiation of THP-1 cells using mass spectrometry. Importin-α1 decreased and importin-α5 increased during the differentiation. Cell cycle-related proteins decreased in both whole cells and nuclei, and proteasome-related proteins increased in the nuclei but not in whole cells. During the differentiation with importin-α1 overexpression, the nuclear levels of some cell division-related proteins recovered, and with importin-α5 knockdown, proteasome assembly factors decreased in the nuclei. In this differentiation, transport receptors reduce unnecessary nuclear proteins by abating import and promoting nuclear proteasomal degradation. This study demonstrates the importance of global nuclear transport control in cell regulation.

Non-transport roles of nuclear import receptors: In need of the right balance

Nuclear import receptors ensure the recognition and transport of proteins across the nuclear envelope into the nucleus. In addition, as diverse processes as mitosis, post-translational modifications at mitotic exit, ciliogenesis, and phase separation, all share a common need for regulation by nuclear import receptors - particularly importin beta-1 and importin beta-2/transportin - independent on nuclear import. In particular, 1) nuclear import receptors regulate the mitotic spindle after nuclear envelope breakdown, 2) they shield cargoes from unscheduled ubiquitination, regulating their timely proteolysis; 3) they regulate ciliary factors, crucial to cell communications and tissue architecture during development; and 4) they prevent phase separation of toxic proteins aggregates in neurons. The balance of nuclear import receptors to cargoes is critical in all these processes, albeit in opposite directions: overexpression of import receptors, as often found in cancer, inhibits cargoes and impairs downstream processes, motivating the therapeutic design of specific inhibitors. On the contrary, elevated expression is beneficial in neuronal contexts, where nuclear import receptors are regarded as potential therapeutic tools in counteracting the formation of aggregates that may cause neurodegeneration. This paradox demonstrates the amplitude of nuclear import receptors-dependent functions in different contexts and adds complexity in considering their therapeutic implications.