Regulation of functional nuclear pore size in fibroblasts.

Protein-NLS-coated gold particles up to approximately 250 A in diameter are transported through the nuclear pores in normal, proliferating BALB/c 3T3 cells. This size can increase or decrease, depending on cellular activity. It has been suggested that increases in functional pore size are related to a reduction in the amount of available p53. To further test this hypothesis, we investigated the effects of cycloheximide and pifithrin-alpha, which inhibits p53-dependent transcriptional activation, on nuclear transport. After 3 hours in cycloheximide, there was a significant increase in the size of the gold particles that entered the nucleoplasm. When the incubation period was extended to 6 hours or longer, transport capacity returned to the control level. By using proteasome inhibitors, it was shown that the cycloheximide-dependent increase in functional pore size was due to the inhibition of protein synthesis, consistent with the fact that p53 is a short-lived protein, and requires the activity of at least two different factors. Although cycloheximide increases the functional diameter of the channel available for signal-mediated transport by approximately 60 A, it had no significant effect on either the import rate of small NLS-containing substrates (FITC-BSA-NLS), or passive diffusion of fluorescent-labeled proteins across the envelope. This suggests that changes in transport capacity were not caused by an increase in overall pore diameter but instead are due to a transient increase in pore size that accompanies signal-mediated transport. Pifithrin-alpha also caused an increase in functional pore diameter without altering the import rate of FITC-BSA-NLS, providing further support for the view that p53 can initiate changes in nuclear transport capacity.

Interaction between NTF2 and xFxFG-containing nucleoporins is required to mediate nuclear import of RanGDP.

Nuclear transport factor 2 (NTF2) is a small, homodimeric protein that binds to both RanGDP and xFxFG repeat-containing nucleoporins, such as yeast Nsp1p and vertebrate p62. NTF2 is required for efficient nuclear protein import and has been shown to mediate the nuclear import of RanGDP. We have used the crystal structures of rat NTF2 and its complex with RanGDP to design a mutant, W7A-NTF2, in which the affinity for xFxFG-repeat nucleoporins is reduced while wild-type binding to RanGDP is retained. The 2.5 A resolution crystal structure of W7A-NTF2 is virtually superimposable upon the wild-type protein structure, indicating that the mutation had not introduced a more general conformational change. Therefore, our data suggest that the exposed side-chain of residue 7 is crucial to the interaction between NTF2 and xFxFG repeat-containing nucleoporins. Consistent with its reduced affinity for xFxFG nucleoporins, fluorescently labelled W7A-NTF2 binds less strongly to the nuclear envelope of permeabilized cultured cells than wild-type NTF2 and, when microinjected into Xenopus oocytes, colloidal gold coated with W7A-NTF2 binds less strongly to the central channel of nuclear pore complexes than wild-type NTF2-coated gold. Significantly, W7A-NTF2 only weakly stimulated the nuclear import of fluorescein-labelled RanGDP, providing direct evidence that an interaction between NTF2 and xFxFG repeat-containing nucleoporins is required to mediate the nuclear import of RanGDP.

Signal-mediated nuclear transport in the amoeba.

The evolutionary changes that occur in signal-mediated nuclear transport would be expected to reflect an increasing need to regulate nucleocytoplasmic exchanges as the complexity of organisms increases. This could involve changes in both the composition and structure of the pore complex, as well as the cytosolic factors that mediate transport. In this regard, we investigated the transport process in amoebae (Amoeba proteus and Chaos carolinensis), primitive cells that would be expected to have less stringent regulatory requirements than more complex organisms. Colloidal gold particles, coated with bovine serum albumin (BSA) conjugated with simple (large T) nuclear localization signals (NLSs), bipartite (nucleoplasmin) NLSs or mutant NLSs, were used to assay nuclear import. It was found that in amoebae (1) the diameter of the particles that are able to enter the nucleoplasm is significantly less than in vertebrate cells, (2) the simple NLS is more effective in mediating nuclear import than the bipartite NLS, and (3) the nucleoporins do not appear to be glycosylated. Evidence was also obtained suggesting that, in amoebae, the simple NLS can mediate nuclear export.

Macromolecular exchanges between the nucleus and cytoplasm.

The control of transcription and translation is of fundamental importance in cell biology. In this regard, the nuclear envelope is in a unique position to contribute to the regulation of these events, by directing macromolecular exchanges between the nucleus and cytoplasm. Such exchanges occur through the nuclear pore complexes, mainly by signal-mediated processes. Different signals are required for import and export. Specific cytoplasmic or nuclear receptors initially bind the signal-containing substrate, and the complex subsequently interacts with the pores. Additional factors then assist in translocation across the envelope. Current research is focused mainly on further characterization of transport receptors, translocation factors, as well as components of the nuclear pore complex, i.e., the nucleoporins. The ultimate goal is to understand the molecular interactions that occur among the different components of the transport apparatus, the energy sources for transport, and how variations in transport capacity are generated.

The location of the transport gate in the nuclear pore complex.

Signal-mediated nuclear transport is a gated process that occurs through a central transporter element located within the pore complex. The purpose of this investigation was to identify the region of the transporter that functions as the gate; i.e. the region that restricts passive diffusion of macromolecules through the pores. To accomplish this, small gold particles coated with polyethylene glycol (PEG; total particle diameter 40-70 A) or large PEG-particles (total diameter 110-270 A) were microinjected into the cytoplasm or nucleoplasm of Xenopus oocytes. Since PEG does not contain either nuclear import or export signals, it is assumed that the particles distribute by simple diffusion. The cells were fixed after 5 or 30 minutes and subsequently examined using TEM. The distribution of the particles located adjacent to and within the pore complexes was then mapped. The results obtained at both 5 and 30 minutes after cytoplasmic injections of small gold were basically the same. The particles readily entered the transporter but, on the average, were approximately 11 times more concentrated in the cytoplasmic half of this structure. The opposite distribution was observed following nuclear injections, i.e. the particles that were located in the transporter were approximately 7 times more numerous in the nuclear half. Our data indicate that there is a single transport gate located in the central domain of the transporter that restricts passive diffusion. The large particles that were injected into the cytoplasm migrated to the surface of the pore complex, but entered the transporter less frequently than small gold. Interestingly, the diffusion of large PEG-particles to the surface of the pores following nuclear injection was greatly restricted; however, this was not the case for similar size particles that were coated with protein containing nuclear export signals (NES). The latter results suggest that the NES is not only required for translocation, but also for migration within the nucleoplasm.

Evidence for the existence of a novel mechanism for the nuclear import of Hsc70.

Hsc70 is a multifunctional protein that is capable of shuttling between the nucleus and the cytoplasm. In this study we investigated the signal-mediated nuclear import step, using Xenopus oocytes as a model system. Purified rat hsc70, and hybrid proteins, which contained either the full-length or the mutated forms of rat hsc70 fused with the maltose binding protein, were labeled with 125I and used as transport substrates. In competition experiments, it was found that the nuclear import of neither purified hsc70 nor the full-length fusion protein was inhibited by an excess of SV40 large T or nucleoplasmin nuclear localization signals (NLSs) that were conjugated with BSA. Since hsc70 contains only a single basic domain (246KRKHKKDISENKRAVRR262), which has the characteristics of an NLS, we examined its role in nuclear import. It was determined, by conjugating this sequence with BSA, that it is capable of promoting nuclear import and, therefore, acts as a prototypical basic NLS. However, inactivation of this signal by deleting the first six amino acids (246KRKHKK251) had no effect on hsc70 import. Overall, the present results indicate that hsc70 utilizes a novel import signal and enters the nucleus by a different mechanism than that employed by simple and bipartite NLSs. The novel signal has not been identified, but we have obtained evidence that it is located in the amino terminus of hsc70.

Variations in signal-mediated nuclear transport during the cell cycle in BALB/c 3T3 cells.

Signal-mediated nuclear import was investigated in proliferating BALB/c 3T3 cells 1, 2, 4, 6, 8, 12, 18, and 21 h after the onset of anaphase. Using nucleoplasmin-coated colloidal gold particles to assay transport capacity, it was found that import was greatest at 1 h postanaphase (after complete reformation of the nuclear envelope). At this time, both the relative rate of gold uptake and the functional size of the transport channels were significantly greater than in control cells, which were randomly selected from nonsynchronized cultures. At 21 h, there was a decrease in the size of the transport channel, but not the rate of nuclear import. This suggests that these two factors are regulated independently. No changes in transport capacity, compared to controls, were detected at other times in the cell cycle.

Regulation of nuclear transport in proliferating and quiescent cells.

Previously, we compared signal-mediated nuclear transport in proliferating and quiescent BALB/c 3T3 cells and found that both the relative rate of nuclear uptake and the functional size of the transport channels were significantly greater in proliferating cells. In this study, the possible causes of these permeability differences were investigated. To determine if the decrease in transport capacity in quiescent cells was due to a reduction in the availability of soluble cytoplasmic factors (i.e., ATP or receptors for nuclear location sequences), or changes in the properties of the pores themselves, proliferating and quiescent cells were fused, and nuclear import of nucleoplasmin-coated gold (NP-gold) particles was assayed in the heterokaryons 50-60 min later. Significant differences in nuclear uptake were maintained following fusion, even though the two nuclei shared a common cytoplasm, consistent with the view that permeability is regulated at the level of the pores. Cell shape also influenced signal-mediated nuclear import. This was demonstrated by studying transport in rounded and flattened cells attached to different-size palladium domains that were deposited on a nonadhesive substrate. Based on analysis of the nuclear uptake rates of large (110-270 A in diameter) and small (50-80 A in diameter) coated gold particles, it was determined that the functional size of the pores was significantly greater in flattened cells. The effect of growth factors on recovery of nuclear transport capacity following serum depletion was also analyzed. Partial recovery was achieved by treating cells with physiological concentrations of EGF, IGF-1, or PDGF; however, complete recovery required both EGF and IGF-1.

Signal-mediated nuclear transport in simian virus 40-transformed cells is regulated by large tumor antigen.

Transformation of cultured cells with simian virus 40 (SV40), or transfection with the early region of the SV40 genome, causes a significant increase in both the rate of signal-mediated nuclear transport and the functional size of the transport channels (located in the pore complexes). By microinjecting purified large tumor (T) antigen into the cytoplasm of murine BALB/c 3T3 cells, we have demonstrated that this protein alone can account for the increase in transport capacity. The T antigen-dependent changes can be partially inhibited by cycloheximide and require a functional nuclear localization sequence. Although necessary, the nuclear localization sequence by itself cannot produce the observed variations in nuclear permeability and presumably function in a "helper" capacity, in association with another, as yet unidentified domain.

The effect of carboxyl-terminal deletions on the nuclear transport rate of rat hsc70.

Rat brain hsc70 is a constitutively expressed member of the 70-kDa family of heat shock proteins that is capable of bidirectional transport across the nuclear envelope when microinjected into Xenopus oocytes [1]. The objective of this study was to identify domains involved in its bidirectional transport. Limited proteolytic digestion with chymotrypsin generated three major truncated proteins of approximately 67.5, 59.5, and 56.5 kDa. Reactivity with NH2-terminal-specific antibodies showed that carboxyl-terminal fragments were removed. Nuclear uptake studies were performed by microinjecting 125I-labeled proteins into the cytoplasm and determining their subsequent nucleocytoplasmic distribution. The accumulation rates, while faster than bovine serum albumin controls, were inversely related to the size of the truncated proteins and greatly reduced compared to undigested hsc70. Nuclear efflux was assayed by microinjecting labeled proteins directly into oocyte nuclei. The relative efflux rates of the truncated polypeptides were less than the undigested protein, and, as observed for uptake, were inversely related to size. These results indicate that the carboxyl-terminal domain of hsc70 is involved in its bidirectional exchange.

Signal-mediated nuclear transport in proliferating and growth-arrested BALB/c 3T3 cells.

Mediated transport across the nuclear envelope was investigated in proliferating and growth-arrested (confluent or serum starved) BALB/c 3T3 cells by analyzing the nuclear uptake of nucleoplasmin-coated colloidal gold after injection into the cytoplasm. Compared with proliferating cells the nuclear uptake of large gold particles (110-270 A in diameter, including the protein coat) decreased 5.5-, 33-, and 78-fold, respectively, in 10-, 14-17-, and 21-d-old confluent cultures; however, the relative uptake of small particles (total diameter 50-80 A) did not decrease with increasing age of the cells. This finding suggests that essentially all pores remain functional in confluent populations, but that most pores lose their capacity to transport large particles. By injecting intermediate-sized gold particles, the functional diameters of the transport channels in the downgraded pores were estimated to be approximately to 130 and 110 A, in 14-17- and 21-d-old cultures, respectively. In proliferating cells, the transport channels have a functional diameter of approximately 230 A. The mean diameters of the pores (membrane-to-membrane distance) in proliferating and confluent cells (728 and 712 A, respectively) were significantly different at the 10%, but not the 5%, level. No differences in pore density (pore per unit length of membrane) were detected. Serum-deprived cells (7-8 d in 1% serum or 4 d in 0.5% serum) also showed a significant decrease in the nuclear uptake of large, but not small, gold particles. Thus, the permeability effects are not simply a function of high cell density but appear to be growth related. The possible functional significance of these findings is discussed.

Identification of two HSP70-related Xenopus oocyte proteins that are capable of recycling across the nuclear envelope.

Two 70-kD polypeptides, B3 and B4, are present in equivalent concentrations in the nucleus and cytoplasm of Xenopus oocytes. The objectives of this study were to determine if they (a) are members of the 70-kD family of heat shock proteins, and (b) recycle between the nuclear and cytoplasmic compartments. Evidence based on high-affinity binding to ATP, cross-reactivity of B3/B4-specific antibodies with rat hsc70, and a comparison of cyanogen bromide cleavage peptide maps with hsc70, verified that B3 and B4 are members of the 70-kD family of heat-shock proteins. Nuclear uptake studies were performed by microinjecting 125I-labeled B3/B4, rat hsc70, and BSA into the cytoplasm of oocytes, and examining their subsequent intracellular distributions. By 6 h postinjection, the nuclear concentration of B3/B4 and hsc70 were approximately 24-fold greater than BSA controls. It was also found that B3/B4-coated gold particles as large as 120A in diameter were able to enter the nucleus by passing through the pores. Nuclear efflux was analyzed by microinjecting the iodinated proteins directly into the oocyte nuclei. 2 h after nuclear injection, at least 46% of the B3/B4 and 60% of the hsc70 were found in the cytoplasmic fractions, compared with less than 10% for the BSA controls. Cell fusion experiments, in which labeled, anucleate oocyte vegetal hemispheres were fused, under oil, with nucleate unlabeled animal hemispheres, demonstrated that cytoplasmic B3 and B4 could enter the nucleus after equilibration was reached, arguing against the existence of separate nuclear and cytoplasmic populations. Collectively, these results show that B3, B4, and rat hsc70 are transported across the nuclear envelope and recycle between the nucleus and cytoplasm.

The permeability of the nuclear envelope in dividing and nondividing cell cultures.

The objective of this study was to determine whether the permeability characteristics of the nuclear envelope vary during different phases of cellular activity. Both passive diffusion and signal-mediated transport across the envelope were analyzed during the HeLa cell cycle, and also in dividing, confluent (growth-arrested), and differentiated 3T3-L1 cultures. Colloidal gold stabilized with BSA was used to study diffusion, whereas transport was investigated using gold particles coated with nucleoplasmin, a karyophilic Xenopus oocyte protein. The gold tracers were microinjected into the cytoplasm, and subsequently localized within the cells by electron microscopy. The rates of diffusion in HeLa cells were greatest during the first and fifth hours after the onset of anaphase. These results correlate directly with the known rates of pore formation, suggesting that pores are more permeable during or just after reformation. Signal-mediated transport in HeLa cells occurs through channels that are located within the pore complexes and have functional diameters up to 230-250 A. Unlike diffusion, no significant differences in transport were observed during different phases of the cell cycle. A comparison of dividing and confluent 3T3-L1 cultures revealed highly significant differences in the transport of nucleoplasmin-gold across the envelope. The nuclei of dividing cells not only incorporated larger particles (230 A versus 190 A in diameter, including the protein coat), but the relative uptake of the tracer was about seven times greater than that in growth-arrested cells. Differentiation of confluent cells to adipocytes was accompanied by an increase in the maximum diameter of the transport channel to approximately 230 A.

Comparison of diverse transport signals in synthetic peptide-induced nuclear transport.

Several investigations have demonstrated the ability of synthetic peptides homologous to the nuclear transport signal of simian virus 40 large T antigen to induce the nuclear transport of nonnuclear carrier proteins. To determine the generality of peptide-induced transport, six peptides with sequences derived from four previously identified nuclear transport signals were synthesized and examined for their ability to induce the transport of mouse immunoglobulin G following microinjection into the cytoplasm of mammalian cells. Peptides containing transport signals from simian virus 40 T antigen, Xenopus nucleoplasmin, and adenovirus E1A proteins were highly efficient at peptide-induced transport, while a peptide homologous to yeast MAT alpha 2 protein was incapable of inducing transport. A short nucleoplasmin peptide that contained only the basic amino acid domain was capable of inducing transport but yielded a much slower rate of transport than a long nucleoplasmin peptide encompassing the previously identified minimal transport signal. The short nucleoplasmin signal exhibited a greater capacity for transport than a peptide homologous to the cytoplasmic mutant T antigen signal when conjugates with a low number of signals coupled per carrier protein were examined. However, the short nucleoplasmin peptide was only marginally more effective than the T antigen mutant peptide when conjugates with a high number of signals coupled per carrier protein were examined.

EM visualization of nucleocytoplasmic transport processes.

The nuclear envelope is strategically located between the nucleoplasm and cytoplasm, and, as such, can play a major role in controlling cellular activity by regulating the exchange of macromolecules between these two compartments. The nuclear pore complexes, which are located within circular areas formed by fusion of the inner and outer membranes of the envelope, represent the primary, if not the exclusive, exchange sites. Individual pores are able to function in both protein import and RNA efflux from the nucleus. Translocation of macromolecules occurs by either passive diffusion or facilitated transport through central channels within the pores. The functional size of the diffusion channel is approximately 9 to over 12 nm in diameter depending on the cell type. The width of the transport channel varies as a function of the number and effectiveness of the specific nuclear targeting signals contained within the permeant molecule. The maximum diameter of the channel can be over 26 nm. Nucleocytoplasmic exchanges can be regulated either by (1) differences in the properties of the transported molecule (molecular size and signal content) or (2) changes in the properties of the pore complexes, which can effect both diffusion and transport.

Nuclear protein synthesis in animal and vegetal hemispheres of Xenopus oocytes.

Experiments were conducted to determine if nuclear proteins are preferentially synthesized in the vicinity of the nucleus, a factor which could facilitate nucleocytoplasmic exchange. Using Xenopus oocytes, animal and vegetal hemispheres were separated by bisecting the cells in paraffin oil. It was initially established that protein synthesis is not affected by the bisecting procedure. To determine if nuclear protein synthesis is restricted to the animal hemisphere (which contains the nucleus), vegetal halves and enucleated animal halves were injected with [3H]leucine and incubated in oil for 90 min. The labeled cell halves were then fused with unlabeled, nucleated animal hemispheres that had been previously injected with puromycin in amounts sufficient to prevent further protein synthesis. Thus, labeled polypeptides which subsequently entered the nuclei were synthesized before fusion. Three hours after fusion, the nuclei were isolated, run on two-dimensional gels, and fluorographed. Approximately 200 labeled nuclear polypeptides were compared, and only 2 were synthesized in significantly different amounts in the animal and vegetal hemispheres. The results indicate that nuclear protein synthesis is not restricted to the cytoplasm adjacent to the nucleus.

The effects of variations in the number and sequence of targeting signals on nuclear uptake.

To determine if the number of targeting signals affects the transport of proteins into the nucleus, Xenopus oocytes were injected with colloidal gold particles, ranging in diameter from 20 to 280 A, that were coated with BSA cross-linked with synthetic peptides containing the SV-40 large T-antigen nuclear transport signal. Three BSA conjugate preparations were used; they had an average of 5, 8, and 11 signals per molecule of carrier protein. In addition, large T-antigen, which contains one signal per monomer, was used as a coating agent. The cells were fixed at various times after injection and subsequently analyzed by electron microscopy. Gold particles coated with proteins containing the SV-40 signal entered the nucleus through central channels located within the nuclear pores. Analysis of the intracellular distribution and size of the tracers that entered the nucleus indicated that the number of signals per molecule affect both the relative uptake of particles and the functional size of the channels available for translocation. In control experiments, gold particles coated with BSA or BSA conjugated with inactive peptides similar to the SV-40 transport signal were virtually excluded from the nucleus. Gold particles coated with nucleoplasmin, an endogenous karyophilic protein that contains five targeting signals per molecule, was transported through the nuclear pores more effectively than any of the BSA-peptide conjugates. Based on a correlation between the peri-envelope density of gold particles and their relative uptake, it is suggested that the differences in the activity of the two targeting signals is related to their binding affinity for envelope receptors. It was also determined, by performing coinjection experiments, that individual pores are capable of recognizing and transporting proteins that contain different nuclear targeting signals.

Translocation of RNA-coated gold particles through the nuclear pores of oocytes.

In the present study, various sized gold particles coated with tRNA, 5S RNA, or poly(A) were used to localize and characterize the pathways for RNA translocation to the cytoplasm. RNA-coated gold particles were microinjected into the nucleus of Xenopus oocytes. The cells were fixed after 15, 60 min, or 6 h, and the particle distribution was later observed by electron microscopy. Similar results were obtained with all classes of RNA used. After nuclear injection, particles ranging from 20-230 A in diameter were observed within central channels of the nuclear pores and in the cytoplasm immediately adjacent to the pores. Particles of this size would not be expected to diffuse through the pores, suggesting that some form of mediated transport occurred. In addition, it was found that the translocation process is saturable. At least 97% of the pores analyzed appeared to be involved in the translocation process. Gold coated with nonphysiological polynucleotides (poly[I] or poly[dA]) were also translocated. When nuclei were injected with either BSA-, ovalbumin-, polyglutamic acid-, or PVP-coated gold, the particles were essentially excluded from the pores. These results indicate that the accumulation of RNA-gold within the pores and adjacent cytoplasm was not due to non-specific effects. We conclude that the translocation sites for gold particles coated with different classes of RNA are located in the centers of the nuclear pores and that particles at least 230 A in diameter can cross the envelope. Tracer particles injected into the cytoplasm were observed within the nuclear pores in areas near the site of injection. However, only a small percentage of the particles actually entered the nucleus. It was also determined, by performing double injection experiments, that individual pores are bifunctional, that is, capable of transporting both proteins and RNA.