A novel transferable nuclear export signal mediates CRM1-independent nucleocytoplasmic shuttling of the human cytomegalovirus transactivator protein pUL69

Abstract

The best studied nuclear export processes are mediated by classical leucine-rich nuclear export signals that specify recognition by the CRM1 export receptor. However, details concerning alternative nuclear export signals and pathways are beginning to emerge. Within the family of Herpesviridae, a set of homologous regulatory proteins that are exemplified by the ICP27 of herpes simplex virus were described recently as nucleocytoplasmic shuttling proteins. Here we report that pUL69 of the beta-herpesvirus human cytomegalovirus is a nuclear protein that is able to shuttle between the nucleus and the cytoplasm independently of virus-encoded cofactors. In contrast to proteins containing a leucine-rich export signal, the shuttling activity of pUL69 was not affected by leptomycin B, indicating that pUL69 trafficking is not mediated by the export receptor CRM1. Importantly, we identified and characterized a novel type of transferable, leptomycin B-insensitive export signal that is distinct from other export signals described previously and is required for pUL69-mediated activation of gene expression. These data suggest that pUL69 is exported via a novel nuclear export pathway, based on a so far unique nuclear export signal of 28 amino acids.

A Novel CRM1-mediated Nuclear Export Signal Governs Nuclear Accumulation of Glyceraldehyde-3-phosphate Dehydrogenase following Genotoxic Stress

“…There are several classes of NES, with typical NES containing hydrophobic amino acids, usually a set of leucine residues (26). Direct interaction with the importin-␤-related export factor CRM1 (exportin 1) is typically essential for export of proteins containing a leucine rich NES (27)(28)(29). Leptomycin B (LMB) is a specific inhibitor of CRM1-mediated nuclear export, inactivating CRM1 by covalent modification of a cysteine residue in the central domain of the polypeptide (30), thereby altering the three-dimensional structure of the CRM1 protein (30,31).…”

Human cytomegalovirus UL69 protein facilitates translation by associating with the mRNA cap-binding complex and excluding 4EBP1

“…pUL69 shuttles between nucleus and cytoplasm (16), binds RNA (17), and interacts with the DExD/Hbox RNA helicase UAP56 or the related URH49 protein (12), cellular proteins involved in RNA transport. The pUL69 transport function is stimulated by both cellular cyclin-dependent kinases (6) and pUL97 (9).…”

The Varicella-Zoster virus IE4 protein: A conserved member of the herpesviral mRNA export factors family and a potential alternative target in antiherpetic therapies

“…Similarly, although the export of ICP27 was first reported to be sensitive to LMB [13,14], the leucinerich N-terminal sequence that is required for an efficient ICP27 export was further shown to function as a CRM-1-independent NES [15,16]. Singularly, a novel type of LMB-insensitive export signal was identified within the UL69 unique C-terminal region [6]. This 28-amino acid domain is not a leucine-rich one and the export pathway used by this unique NES has not yet been defined.…”

“…All members of this protein family have been reported to shuttle between the nucleus and the cytoplasm independently of virus-encoded cofactors indicating that it is an intrinsic characteristic [4][5][6][7]. Generally, nucleocytoplasmic protein trafficking occurs through direct interactions between the transport signals NLS and NES on the proteins and import or export receptors that mediate passage through the nuclear pore complex (NPC).…”

“…In contrast to ICP27, UL69 localizes within the nucleus but is excluded from the nucleolus [2]. It was also shown to contain an arginine-rich region at the N-terminus which resembles a bipartite NLS [6]. Similarly, EB2 was shown to contain two KH-rich motifs, mapping to residues 127-130 and 143-145, which function as two independent NLS sequences [9].…”