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Structure of HCMV Small Terminase NLS bound to importin alpha
Structural highlights
FunctionIMA1_MOUSE Functions in nuclear protein import as an adapter protein for nuclear receptor KPNB1. Binds specifically and directly to substrates containing either a simple or bipartite NLS motif. Docking of the importin/substrate complex to the nuclear pore complex (NPC) is mediated by KPNB1 through binding to nucleoporin FxFG repeats and the complex is subsequently translocated through the pore by an energy requiring, Ran-dependent mechanism. At the nucleoplasmic side of the NPC, Ran binds to importin-beta and the three components separate and importin-alpha and -beta are re-exported from the nucleus to the cytoplasm where GTP hydrolysis releases Ran from importin. The directionality of nuclear import is thought to be conferred by an asymmetric distribution of the GTP- and GDP-bound forms of Ran between the cytoplasm and nucleus. Publication Abstract from PubMedThe tripartite terminase complex of herperviruses assembles in the cytoplasm of infected cells and exploits the host nuclear import machinery to gain access to the nucleus, where capsid assembly and genome-packaging occur. Here, we analyzed the structure and conservation of Nuclear Localization Signal (NLS) sequences previously identified in Herpes Simplex Virus 1 (HSV-1) large terminase and Human Cytomegalovirus (HCMV) small terminase. We found a monopartite NLS at the N-terminus of large terminase, flanking the ATPase domain, that is conserved only in alpha-herpesviruses. In contrast, small terminase exposes a classical NLS at the far C-terminus of its helical structure, which is conserved only in two genera of the beta-subfamily and absent in alpha- and gamma-herpesviruses. In addition, we predicted a classical NLS in the third terminase subunit that is partially conserved among herpesviruses. Bioinformatic analysis revealed that both location and potency of NLSs in terminase subunits evolved more rapidly than the rest of the aminoacid sequence, despite the selective pressure to keep terminase gene products active and localized in the nucleus. We propose that swapping NLSs among terminase subunits is a regulatory mechanism that allows different herpesviruses to regulate the kinetics of terminase nuclear import, reflecting a mechanism of virus:host adaptation. Divergent Evolution of Nuclear Localization Signal Sequences in Herpesvirus Terminase Subunits.,Sankhala RS, Lokareddy RK, Cingolani G J Biol Chem. 2016 Mar 31. pii: jbc.M116.724393. PMID:27033706[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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