Alternative titles; symbols
HGNC Approved Gene Symbol: KPNA2
Cytogenetic location: 17q24.2 Genomic coordinates (GRCh38): 17:68,035,735-68,046,854 (from NCBI)
The import of proteins into the nucleus is a process that involves at least 2 steps. The first is an energy-independent docking of the protein to the nuclear envelope and the second is an energy-dependent translocation through the nuclear pore complex. Imported proteins require a nuclear localization sequence (NLS) which generally consists of a short region of basic amino acids or 2 such regions spaced about 10 amino acids apart. Proteins involved in the first step of nuclear import have been identified in different systems. These include the Xenopus protein importin and its yeast homolog, SRP1 (a suppressor of certain temperature-sensitive mutations of RNA polymerase I in Saccharomyces cerevisiae; see 600686), which bind to the NLS. To find proteins that interact with the recombination-activating protein RAG1 (179615), Cuomo et al. (1994) used a 2-hybrid assay and cloned a cDNA, which they referred to as Rch1 for 'Rag cohort-1' and which was later shown to be SRP1-alpha. They found that Rch1, along with RAG2 (179616), induced V(D)J recombinase activity in a variety of nonlymphoid cell types.
The SRP1-like proteins share large blocks of evolutionarily conserved sequence and belong to a multigene family, the members of which may be involved in the nuclear transport of a variety of proteins. Using a yeast 2-hybrid system with a known nuclear protein as bait, Weis et al. (1995) captured a cDNA from HeLa cells which encodes an NLS receptor. The full-length cDNA encodes a putative 529-amino acid protein and was termed SRP1-alpha based on its high homology to the yeast SRP1 gene. Human SRP1-alpha was shown to bind to the NLS and to mediate the docking step. When both SRP1-alpha and Ran (601179), another protein implicated in the transport mechanism, were used in an in vitro assay system, Weis et al. (1995) were able to reconstitute the nuclear transport machinery in vitro. This gene is also symbolized KPNA2 for karyopherin alpha-2.
Epstein-Barr virus (EBV) is implicated in the induction of several malignancies. Fischer et al. (1997) used a yeast 2-hybrid system to show that karyopherin alpha-2 forms a complex with the EBV nuclear antigen-1 (EBNA1). Nuclear transport of EBNA1 was impaired by expression of N-terminally truncated KPNA2.
Seki et al. (1997) demonstrated that KPNA2, which they called QIP2, interacted with the NLSs of DNA helicase Q1 (RECQL; 600537) and SV40 T antigen. QIP2 contains 'armadillo' repeats, which are 42-amino acid motifs implicated in protein-protein interactions. The human QIP2 protein is 63%, 50%, 45%, and 44% identical to the Xenopus importin-alpha, human QIP1 (602970), S. cerevisiae SRP1, and human KPNA1 (600686) proteins, respectively.
Using an in vitro import assay based on permeabilized HeLa cells to study the import substrate specificity of all ubiquitously expressed importins, including KPNA2, Kohler et al. (1999) found that all importins tested were able to transport HNRNPK (600712) and PCAF (602303), in addition to the standard test substrates, but only KPNA4 (601892) showed a strong preference for the import of GDP/GTP exchange factor RCC1 (179710), which is exclusively located inside the nucleus. When HNRNPK, PCAF, and RCC1 were offered with a competing substrate nucleoplasmin (164040), they found that substrate binding was diminished or abolished in some importins and retained in others.
Using immunoprecipitation analysis in mouse and human cells, Cai et al. (2020) showed that ubiquitin-specific protease-22 (USP22; 612116) interacted with the transcription factor IRF3 (603734) in cytoplasm after viral infection. Domain mapping analysis suggested that the C-terminal ubiquitin peptidase domain of USP22 was responsible for its association with IRF3. Knockdown of USP22 in human cell lines inhibited IRF3 nuclear accumulation. Analysis with Usp22 -/- mouse embryonic fibroblasts (MEFs) confirmed that Usp22 was essential for virus-triggered nuclear translocation of Irf3 and for subsequent cellular antiviral responses. Consequently, loss of Usp22 in mice led to increased susceptibility to viral infection. Usp22 was required for optimal induction of type I interferons after viral infection in mice, and Irf3 nuclear accumulation and antiviral signaling required Usp22 deubiquitinating activity. However, Usp22 did not directly target Irf3 for deubiquitination, but instead regulated Irf3 nuclear translocation by directly targeting the intermediate protein Kpna2 for deubiquitination. Kpna2 interacted constitutively with Usp22 through the C-terminal peptidase domain of Usp22, and it interacted with Irf3 or phosphorylated Irf3 in a viral infection-dependent manner. Deubiquitination of Kpna2 by Usp22 inhibited Kpna2 degradation, which promoted of virus-triggered signaling by facilitating nuclear translocation of Irf3.
Dorr et al. (2001) presented the genomic organization of the KPNA2 gene with 11 exons spanning approximately 10 kb.
Dorr et al. (2001) localized KPNA2 in close proximity to a translocation breakpoint on chromosome 17q23-q24.
Crystal Structure
Matsuura and Stewart (2004) presented the 2.0-angstrom crystal structure of the nuclear export complex formed by exportin Cse1p (see 601342) complexed with its cargo Kap60p (importin-alpha) and RanGTP (see 602362), thereby providing a structural framework for understanding nuclear protein export and the different functions of RanGTP in export and import. In the complex, Cse1p coils around both RanGTP and Kap60p, stabilizing the RanGTP-state and clamping the Kap60p importin-beta (see 602738)-binding domain, ensuring that only cargo-free Kap60p is exported. By mutagenesis, Matsuura and Stewart (2004) showed that conformational changes in exportins couple cargo binding to high affinity for RanGTP, generating a spring-loaded molecule to facilitate disassembly of the export complex following GTP hydrolysis in the cytoplasm.
Exclusion Studies
The KPNA2 gene is located within a candidate region for Silver-Russell syndrome (SRS; 180860) (Dorr et al., 2001). However, in a genomic screen for mutations within all exons and adjacent intronic sequences of KPNA2 from 31 unrelated SRS patients, Dorr et al. (2001) identified several single-nucleotide polymorphisms (SNPs) but no disease-related mutation.
Cai, Z., Zhang, M.-X., Tang, Z., Zhang, Q., Ye, J., Xiong, T.-C., Zhang, Z.-D., Zhong, B. USP22 promotes IRF3 nuclear translocation and antiviral responses by deubiquitinating the importin protein KPNA2. J. Exp. Med. 217: e20191174, 2020. Note: Electronic Article. [PubMed: 32130408] [Full Text: https://doi.org/10.1084/jem.20191174]
Cuomo, C. A., Kirch, S. A., Gyuris, J., Brent, R., Oettinger, M. A. Rch1, a protein that specifically interacts with the RAG-1 recombination-activating protein. Proc. Nat. Acad. Sci. 91: 6156-6160, 1994. [PubMed: 8016130] [Full Text: https://doi.org/10.1073/pnas.91.13.6156]
Dorr, S., Midro, A. T., Farber, C., Giannakudis, J., Hansmann, I. Construction of a detailed physical and transcript map of the candidate region for Russell-Silver syndrome on chromosome 17q23-q24. Genomics 71: 174-181, 2001. [PubMed: 11161811] [Full Text: https://doi.org/10.1006/geno.2000.6413]
Dorr, S., Schlicker, M., Hansmann, I. Genomic structure of karyopherin alpha-2 (KPNA2) within a low-copy repeat on chromosome 17q23-q24 and mutation analysis in patients with Russell-Silver syndrome. Hum. Genet. 109: 479-486, 2001. [PubMed: 11735022] [Full Text: https://doi.org/10.1007/s004390100605]
Fischer, N., Kremmer, E., Lautscham, G., Mueller-Lantzsch, N., Grasser, F. A. Epstein-Barr virus nuclear antigen 1 forms a complex with the nuclear transporter karyopherin alpha-2. J. Biol. Chem. 272: 3999-4005, 1997. [PubMed: 9020106] [Full Text: https://doi.org/10.1074/jbc.272.7.3999]
Kohler, M., Speck, C., Christiansen, M., Bischoff, F. R., Prehn, S., Haller, H., Gorlich, D., Hartmann, E. Evidence for distinct substrate specificities of importin alpha family members in nuclear protein import. Molec. Cell. Biol. 19: 7782-7791, 1999. [PubMed: 10523667] [Full Text: https://doi.org/10.1128/MCB.19.11.7782]
Matsuura, Y., Stewart, M. Structural basis for the assembly of a nuclear export complex. Nature 432: 872-877, 2004. [PubMed: 15602554] [Full Text: https://doi.org/10.1038/nature03144]
Seki, T., Tada, S., Katada, T., Enomoto, T. Cloning of a cDNA encoding a novel importin-alpha homologue, Qip1: discrimination of Qip1 and Rch1 from hSrp1 by their ability to interact with DNA helicase Q1/RecQL. Biochem. Biophys. Res. Commun. 234: 48-53, 1997. [PubMed: 9168958] [Full Text: https://doi.org/10.1006/bbrc.1997.6535]
Weis, K., Mattaj, I. W., Lamond, A. I. Identification of hSRP1-alpha as a functional receptor for nuclear localization sequences. Science 268: 1049-1052, 1995. [PubMed: 7754385] [Full Text: https://doi.org/10.1126/science.7754385]