Abstract
BackgroundClassical nuclear localization signal (NLS) dependent nuclear import is carried out by a heterodimer of importin α and importin β. NLS cargo is recognized by importin α, which is bound by importin β. Importin β mediates translocation of the complex through the central channel of the nuclear pore, and upon reaching the nucleus, RanGTP binding to importin β triggers disassembly of the complex. To date, six importin α family members, encoded by separate genes, have been described in humans.ResultsWe sequenced and characterized a seventh member of the importin α family of transport factors, karyopherin α 7 (KPNA7), which is most closely related to KPNA2. The domain of KPNA7 that binds Importin β (IBB) is divergent, and shows stronger binding to importin β than the IBB domains from of other importin α family members. With regard to NLS recognition, KPNA7 binds to the retinoblastoma (RB) NLS to a similar degree as KPNA2, but it fails to bind the SV40-NLS and the human nucleoplasmin (NPM) NLS. KPNA7 shows a predominantly nuclear distribution under steady state conditions, which contrasts with KPNA2 which is primarily cytoplasmic.ConclusionKPNA7 is a novel importin α family member in humans that belongs to the importin α2 subfamily. KPNA7 shows different subcellular localization and NLS binding characteristics compared to other members of the importin α family. These properties suggest that KPNA7 could be specialized for interactions with select NLS-containing proteins, potentially impacting developmental regulation.
Highlights
Classical nuclear localization signal (NLS) dependent nuclear import is carried out by a heterodimer of importin a and importin b
We used BLAST to query the human genome with the importin a family member KPNA2, and retrieved a sequence predicted to encode a seventh member of the importin a family
We sequenced the cDNA [Genbank:EU126604], and found that it matched that of the predicted gene, leading us to conclude that the gene is transcribed in human cells
Summary
Classical nuclear localization signal (NLS) dependent nuclear import is carried out by a heterodimer of importin a and importin b. Importin b mediates translocation of the complex through the central channel of the nuclear pore, and upon reaching the nucleus, RanGTP binding to importin b triggers disassembly of the complex. Transport of proteins between the nucleus and the cytoplasm is carried out by karyopherins, a family of proteins made up of importins and exportins [2,3]. Classical nuclear localization signal (NLS) dependent nuclear import is carried out by importin a and importin b Importin a family members bind NLS cargo, and bind to importin b through an Nterminal importin b binding domain (IBB). Importin b mediates translocation of the NLS-Importin a-Importin b import complex into the nucleus through direct interactions with the NPC.
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