The full-length interleukin-33 (FLIL33)--importin-5 interaction does not regulate FLIL33's nuclear localization but controls its intracellular degradation.

5 years ago

The full-length interleukin-33 (FLIL33)--importin-5 interaction does not regulate FLIL33's nuclear localization but controls its intracellular degradation.

Brian S Hampton, Rita Fishelevich, Zahid Noor, Nevins W Todd, Virginia Lockatell, Andrew Clerman, Sergei P Atamas, Nirav G Shah, Irina G Luzina, Mariah V Salcedo

Human mature interleukin-33 (MIL33) is a member of the IL-1 family and a potent regulator of immunity through its pro-T helper cell 2 (Th2) activity. Its precursor form, full-length interleukin-33 (FLIL33), is an intranuclear protein in many cell types, including fibroblasts, and its intracellular levels can change in response to stimuli. However, the mechanisms controlling the nuclear localization of FLIL33 or its stability in cells are not understood. Here, we identified importin-5 (IPO5), a member of the importin family of nuclear transport proteins, as an intracellular binding partner of FLIL33. By overexpressing various FLIL33 protein segments and variants in primary human lung fibroblasts and HEK293T cells, we show that FLIL33, but not MIL33, physically interacts with IPO5 and that this interaction localizes to a cluster of charged amino acids (46-56) but not to an adjacent segment (61-67) in the FLIL33 N-terminal region. siRNA-mediated IPO5 knockdown in cell culture did not affect FLIL33's nuclear localization. However, the IPO5 knockdown significantly decreased the intracellular levels of overexpressed FLIL33, reversed by treatment with the 20S proteasome inhibitor bortezomib. Furthermore, FLIL33 variants deficient in IPO5 binding remained intranuclear and exhibited decreased levels, which were also restored by the bortezomib treatment. These results indicate that the interaction between FLIL33 and IPO5 is localized to a specific segment of the FLIL33 protein, is not required for nuclear localization of FLIL33, and protects FLIL33 from proteasome-dependent degradation.

Publisher URL: http://doi.org/10.1074/jbc.M117.807636

DOI: 10.1074/jbc.M117.807636