Importin-alpha-16 is a translocon-associated protein involved in sorting membrane proteins to the nuclear envelope

Functional association of Sun1 with nuclear pore complexes

Sun1 and 2 are A-type lamin-binding proteins that, in association with nesprins, form a link between the inner nuclear membranes (INMs) and outer nuclear membranes of mammalian nuclear envelopes. Both immunofluorescence and immunoelectron microscopy reveal that Sun1 but not Sun2 is intimately associated with nuclear pore complexes (NPCs). Topological analyses indicate that Sun1 is a type II integral protein of the INM. Localization of Sun1 to the INM is defined by at least two discrete regions within its nucleoplasmic domain. However, association with NPCs is dependent on the synergy of both nucleoplasmic and lumenal domains. Cells that are either depleted of Sun1 by RNA interference or that overexpress dominant-negative Sun1 fragments exhibit clustering of NPCs. The implication is that Sun1 represents an important determinant of NPC distribution across the nuclear surface.

Retention and mobility of the mammalian lamin B receptor in the plant nuclear envelope

BACKGROUND INFORMATION

In a previous study, we showed that GFP (green fluorescent protein) fused to the N-terminal 238 amino acids of the mammalian LBR (lamin B receptor) localized to the NE (nuclear envelope) when expressed in the plant Nicotiana tabacum. The protein was located in the NE during interphase and migrated with nuclear membranes during cell division. Targeting and retention of inner NE proteins requires several mechanisms: signals that direct movement through the nuclear pore complex, presence of a transmembrane domain or domains and retention by interaction with nuclear or nuclear-membrane constituents.

RESULTS

Binding mutants of LBR-GFP were produced to investigate the mechanisms for the retention of LBR in the NE. FRAP (fluorescence recovery after photobleaching) analysis of mutant and wild-type constructs was employed to examine the retention of LBR-GFP in the plant NE. wtLBR-GFP (wild-type LBR-GFP) was shown to have significantly lower mobility in the NE than the lamin-binding domain deletion mutant, which showed increased mobility in the NE and was also localized to the endoplasmic reticulum and punctate structures in some cells. Modification of the chromatin-binding domain resulted in the localization of the protein in nuclear inclusions, in which it was immobile.

CONCLUSIONS

As expression of truncated LBR-GFP in plant cells results in altered targeting and retention compared with wtLBR-GFP, we conclude that plant cells can recognize the INE (inner NE)-targeting motif of LBR. The altered mobility of the truncated protein suggests that not only do plant cells recognize this signal, but also have nuclear proteins that interact weakly with LBR.

Genomic sequence analysis of a nucleopolyhedrovirus isolated from the diamondback moth, Plutella xylostella

The CL3 plaque isolate of Plutella xylostella multiple nucleopolyhedrovirus (PlxyMNPV-CL3) exhibits a high degree of genetic similarity with the Autographa californica MNPV (AcMNPV) but is significantly more virulent against the diamondback moth, P. xylostella, than AcMNPV. To identify genetic differences between PlxyMNPV-CL3 and AcMNPV that may account for the difference in virulence against P. xylostella, the genome sequence of the CL3 plaque isolate of PlxyMNPV was determined and compared to the genome sequence of AcMNPV isolate C6. The PlxyMNPV genome is 134,417 bp, 523 bp larger than the AcMNPV-C6 genome, and the nucleotide sequence is almost completely co-linear with that of AcMNPV-C6. Of the 153 open reading frames (ORFs) identified in PlxyMNPV, 151 had homologues in AcMNPV-C6, with a mean amino acid sequence identity of 98.5%. The PlxyMNPV genome possessed two features previously reported for other variants of AcMNPV: (1) an extra baculovirus repeated orf (bro) sequence located between the plxy29/ac30 and sod ORFs, and (2) the deletion of the AcMNPV pnk/pnl polynucleotide kinase/ligase gene. In addition, an 817 bp insert of unknown origin located between the fp25K and lef-9 genes was discovered. This insert contained two small ORFs and was detected in both tissue culture- and larvae-derived PlxyMNPV DNA by PCR. Finally, the PlxyMNPV-CL3 ie-2 gene encodes a product with a low level (37.3%) of amino acid sequence identity with the AcMNPV-C6 ie-2 product. PlxyMNPV-CL3 apparently acquired this variant ie2 gene by recombination with an undescribed nucleopolyhedrovirus.

Early sorting of inner nuclear membrane proteins is conserved

Spodoptera frugiperda (Sf9) importin-alpha-16 is a translocon-associated protein that participates in the early sorting pathway of baculovirus integral membrane proteins destined for the inner nuclear membrane (INM). To discern whether sorting intermediate protein complexes like those observed in insect cells are also formed with mammalian INM proteins, cross-linked complexes of importin-alpha-16 with human lamin B receptor (LBR) and nurim were examined. Both LBR and nurim cross-link with Sf9 importin-alpha-16 during cotranslational membrane integration and remain proximal with importin-alpha-16 after integration into the endoplasmic reticulum membrane and release from the translocon. Human cells encode several isoforms of importin-alpha; to determine whether any of these isoforms may recognize INM-directed proteins, they were tested for their ability to cross-link with the viral-derived INM sorting motif sequence. One cross-linked adduct was detected with a 16-kDa isoform encoded from KPNA4 (KPNA-4-16). KPNA-4-16 was easily detected in microsomal membranes prepared from KPNA4-16 recombinant virus-infected cells and was also detected in microsomes prepared from HeLa cells. Together these observations suggest that elements of the early sorting pathway of INM-directed proteins mediated by importin-alpha-16 are highly conserved, and mammalian KPNA-4-16 is a candidate partner in sorting integral membrane proteins to the INM.

Highway to the inner nuclear membrane: rules for the road

To enter the nucleus a protein must be chaperoned by a transport factor through the nuclear pore complex or it must be small enough to pass through by diffusion. Although these principles have long described the nuclear import of soluble proteins, recent evidence indicates that they also apply to the import of integral inner nuclear membrane proteins. Here we develop a set of rules that might govern the transport of proteins to the inner nuclear membrane.

Nuclear localization of functional metabotropic glutamate receptor mGlu1 in HEK293 cells and cortical neurons: role in nuclear calcium mobilization and development

The Group I metabotropic glutamate receptor (mGlu1) plays an important role in neuromodulation, development, and synaptic plasticity. Using immunocytochemistry, subcellular fractionation, and western blot analysis, the present study shows that mGlu1a receptors are present on nuclear membranes in stably transfected human embryonic kidney 293 (HEK293) cells as well as being endogenously expressed on rat cortical nuclei. Both glutamate and the group I agonist, quisqualate, directly activate nuclear mGlu1 receptors leading to a characteristic oscillatory pattern of calcium flux in isolated HEK nuclei and a slow rise to plateau in isolated cortical nuclei. In either case calcium responses could be terminated upon application of the mGlu1-selective antagonist, 7-(hydroxyamino)cyclopropa[b]chromen-1a-carboxylate ethyl ester. Responses could also be blocked by ryanodine and inositol 1,4,5-triphosphate receptor inhibitors, demonstrating the involvement of these calcium channels. Agonist activation of intracellular receptors was driven by Na(+)-dependent and -independent processes in nuclei isolated from either HEK or cortical neurons. Finally, mGlu1 nuclear receptors were dramatically up-regulated in the course of post-natal development. Therefore, like the other Group I receptor, mGlu5, mGlu1 can function as an intracellular receptor, suggesting a more encompassing role for nuclear G protein-coupled receptors and downstream signaling elements in the regulation of nuclear events.

BV/ODV-E26: a palmitoylated, multifunctional structural protein of Autographa californica nucleopolyhedrovirus

Autographa californica nucleopolyhedrovirus Ac16 is 1 of 17 genes conserved within Type 1 nucleopolyhedroviruses. This report demonstrates that multiple isoforms of the protein encoded by Ac16, BV/ODV-E26 (E26), are present in the infected cell. One form of E26 associates with viral DNA or DNA-binding proteins, while a second form associates with intracellular membranes and this is likely due to palmitoylation. The different forms of E26 present unique epitopes that can be discriminated by antiserum produced to bacterially or virally produced antigen. A summation of the data now available on E26 suggests that it is a multifunctional protein and the functional states assume unique conformations that can be discriminated by differing antisera.

SV40 VP2 and VP3 Insertion into ER Membranes Is Controlled by the Capsid Protein VP1: Implications for DNA Translocation out of the ER

Nonenveloped viruses such as Simian Virus 40 (SV40) exploit established cellular pathways for internalization and transport to their site of penetration. By analyzing mutant SV40 genomes that do not express VP2 or VP3, we found that these structural proteins perform essential functions that are regulated by VP1. VP2 significantly enhanced SV40 particle association with the host cell, while VP3 functioned downstream. VP2 and VP3 both integrated posttranslationally into the endoplasmic reticulum (ER) membrane. Association with VP1 pentamers prevented their ER membrane integration, indicating that VP1 controls the function of VP2 and VP3 by directing their localization between the particle and the ER membrane. These findings suggest a model in which VP2 aids in cell binding. After capsid disassembly within the ER lumen, VP3, and perhaps VP2, oligomerizes and integrates into the ER membrane, potentially creating a viroporin that aids in viral DNA transport out of the ER.

Karyopherin-mediated import of integral inner nuclear membrane proteins

Targeting of newly synthesized integral membrane proteins to the appropriate cellular compartment is specified by discrete sequence elements, many of which have been well characterized. An understanding of the signals required to direct integral membrane proteins to the inner nuclear membrane (INM) remains a notable exception. Here we show that integral INM proteins possess basic sequence motifs that resemble 'classical' nuclear localization signals. These sequences can mediate direct binding to karyopherin-alpha and are essential for the passage of integral membrane proteins to the INM. Furthermore, karyopherin-alpha, karyopherin-beta1 and the Ran GTPase cycle are required for INM targeting, underscoring parallels between mechanisms governing the targeting of integral INM proteins and soluble nuclear transport. We also provide evidence that specific nuclear pore complex proteins contribute to this process, suggesting a role for signal-mediated alterations in the nuclear pore complex to allow for passage of INM proteins along the pore membrane.