Identification of new FGF1 binding partners-Implications for its intracellular function

Besides its classical mode of action through activation of specific receptors at the cell surface, fibroblast growth factor 1 (FGF1) can also cross the cellular membrane and translocate into the cytosol and further to the nucleus. The mechanism of this translocation is described partially, but the role of FGF1 inside the cell remains unknown. The aim of our work was to identify novel binding partners of FGF1 to predict its intracellular functions. We combined three methods of identification of such partners based on different principles: yeast two‐hybrid screen and mass spectrometry (MS) analysis of complexes obtained by Tandem Affinity Purification (TAP) or by co‐precipitation from cell lysate using recombinant FGF1. Altogether, we identified twenty novel intracellular proteins interacting with FGF1. For selected proteins, their direct interaction with FGF1 was confirmed by pull‐down assays and SPR measurements. Interestingly, half of the proteins found are involved in processes related to cell viability, such as apoptosis, cell proliferation, and cell cycle regulation. Thus, our study indicates that the role of intracellular FGF1 is to protect the cell against stress conditions by providing an additional signal for cell survival, independently of receptor‐activated signaling cascades. © 2016 IUBMB Life, 68(3):242–251, 2016

Nucleolin regulates phosphorylation and nuclear export of fibroblast growth factor 1 (FGF1)

Extracellular fibroblast growth factor 1 (FGF1) acts through cell surface tyrosine kinase receptors, but FGF1 can also act directly in the cell nucleus, as a result of nuclear import of endogenously produced, non-secreted FGF1 or by transport of extracellular FGF1 via endosomes and cytosol into the nucleus. In the nucleus, FGF1 can be phosphorylated by protein kinase C δ (PKCδ), and this event induces nuclear export of FGF1. To identify intracellular targets of FGF1 we performed affinity pull-down assays and identified nucleolin, a nuclear multifunctional protein, as an interaction partner of FGF1. We confirmed a direct nucleolin-FGF1 interaction by surface plasmon resonance and identified residues of FGF1 involved in the binding to be located within the heparin binding site. To assess the biological role of the nucleolin-FGF1 interaction, we studied the intracellular trafficking of FGF1. In nucleolin depleted cells, exogenous FGF1 was endocytosed and translocated to the cytosol and nucleus, but FGF1 was not phosphorylated by PKCδ or exported from the nucleus. Using FGF1 mutants with reduced binding to nucleolin and a FGF1-phosphomimetic mutant, we showed that the nucleolin-FGF1 interaction is critical for the intranuclear phosphorylation of FGF1 by PKCδ and thereby the regulation of nuclear export of FGF1.

Clathrin- and dynamin-independent endocytosis of FGFR3--implications for signalling

Endocytosis of tyrosine kinase receptors can influence both the duration and the specificity of the signal emitted. We have investigated the mechanisms of internalization of fibroblast growth factor receptor 3 (FGFR3) and compared it to that of FGFR1 which is internalized predominantly through clathrin-mediated endocytosis. Interestingly, we observed that FGFR3 was internalized at a slower rate than FGFR1 indicating that it may use a different endocytic mechanism than FGFR1. Indeed, after depletion of cells for clathrin, internalization of FGFR3 was only partly inhibited while endocytosis of FGFR1 was almost completely abolished. Similarly, expression of dominant negative mutants of dynamin resulted in partial inhibition of the endocytosis of FGFR3 whereas internalization of FGFR1 was blocked. Interfering with proposed regulators of clathrin-independent endocytosis such as Arf6, flotillin 1 and 2 and Cdc42 did not affect the endocytosis of FGFR1 or FGFR3. Furthermore, depletion of clathrin decreased the degradation of FGFR1 resulting in sustained signalling. In the case of FGFR3, both the degradation and the signalling were only slightly affected by clathrin depletion. The data indicate that clathrin-mediated endocytosis is required for efficient internalization and downregulation of FGFR1 while FGFR3, however, is internalized by both clathrin-dependent and clathrin-independent mechanisms.

Roles of fibroblast growth factor receptors in carcinogenesis

The fibroblast growth factor receptors (FGFR) play essential roles both during development and in the adult. Upon ligand binding, FGFRs induce intracellular signaling networks that tightly regulate key biological processes, such as cell proliferation, survival, migration, and differentiation. Deregulation of FGFR signaling can thus alter tissue homeostasis and has been associated with several developmental syndromes as well as with many types of cancer. In human cancer, FGFRs have been found to be deregulated by multiple mechanisms, including aberrant expression, mutations, chromosomal rearrangements, and amplifications. In this review, we will give an overview of the main FGFR alterations described in human cancer to date and discuss their contribution to cancer progression.

Size limitation in translocation of fibroblast growth factor 1 fusion proteins across the endosomal membrane

After binding to its receptor on the surface of mammalian cells and subsequent endocytosis, FGF1 is translocated across the membrane into the cytosol. The growth factor is then further transported into the nucleus. In order to characterize more closely the translocation mechanism utilized by FGF1, we introduced additional amino acids into FGF1 to test the size dependence of the translocated substrate. We constructed mutants containing an increasing number of copies of the myc tag (1-13 copies) in a surface loop of the FGF1 molecule. All of the constructs bound to specific FGF receptors and to heparin and were taken up by endocytosis. However, only FGF1 mutants harboring up to three myc tags (53 amino acids) were translocated while mutants with five myc tags (77 amino acids) or more were not translocated through the membrane. We further showed that insertion of other, unrelated polypeptides into FGF1, i.e., 3xFLAG tag (22 amino acids) and streptavidin binding peptide (50 amino acids), was also translocated. Larger insertions into FGF1, like the CBP-SBP tag (82 amino acids) or ricin A-chain (272 amino acids), resulted in fusion proteins that failed to translocate. The presented data imply that it is possible to employ FGF1 to import various polypeptides into the cytosol and nucleus of cells. Furthermore, the strict size dependence of FGF1 fusion proteins in membrane translocation argues against simple leakage of FGF1 from ruptured endosomal membranes but rather points to a specific translocation apparatus involving a proteinaceous pore.

Increased protein stability of FGF1 can compensate for its reduced affinity for heparin

Human FGF1 (fibroblast growth factor 1) is a powerful signaling molecule with a short half-life in vivo and a denaturation temperature close to physiological. Binding to heparin increases the stability of FGF1 and is believed to be important in the formation of FGF1.fibroblast growth factor receptor (FGFR) active complex. In order to reveal the function of heparin in FGF1.FGFR complex formation and signaling, we constructed several FGF1 variants with reduced affinity for heparin and with diverse stability. We determined their biophysical properties and biological activities as well as their ability to translocate across cellular membranes. Our study showed that increased thermodynamic stability of FGF1 nicely compensates for decreased binding of heparin in FGFR activation, induction of DNA synthesis, and cell proliferation. By stepwise introduction of stabilizing mutations into the K118E (K132E) FGF1 variant that shows reduced affinity for heparin and is inactive in stimulation of DNA synthesis, we were able to restore the full mitogenic activity of this mutant. Our results indicate that the main role of heparin in FGF-induced signaling is to protect this naturally unstable protein against heat and/or proteolytic degradation and that heparin is not essential for a direct FGF1-FGFR interaction and receptor activation.

Ubiquitination of fibroblast growth factor receptor 1 is required for its intracellular sorting but not for its endocytosis

Endocytosis and targeting of growth factor receptors for lysosomal degradation have been associated with ubiquitination of the intracellular part of the receptors. To elucidate the role of receptor ubiquitination in internalization and sorting of fibroblast growth factor receptor (FGFR), we constructed several mutants of FGFR1 in which lysines, potential ubiquitination sites, were substituted for arginines. Substitution of all lysine residues in the intracellular part of FGFR1 resulted in inactivation of the tyrosine kinase domain of the receptor. However, several multilysine FGFR1 mutants, where up to 26 of 29 lysines in the intracellular part of the receptor were mutated, retained tyrosine kinase activity. The active multilysine mutants were poorly ubiquitinated, but internalized normally, indicating that ubiquitination of the receptor is not required for endocytosis. In contrast, degradation of the multilysine mutants was dramatically reduced as the mutants were inefficiently transported to lysosomes but rather sorted to recycling endosomes. The altered sorting resulted in sustained signaling. The duration of FGFR1 signaling seems to be tightly regulated by receptor ubiquitination and subsequent sorting to the lysosomes for degradation.

Structural requirements of FGF-1 for receptor binding and translocation into cells

FGF-1 binds to and activates specific transmembrane receptors (FGFRs) and is subsequently internalized and translocated to the interior of the cell. To elucidate the role of the receptor in the translocation process, we studied the effects of the elimination of distinct sites of the ligand-receptor interaction. On the basis of the structure of the FGF-1-FGFR1 complex, we substituted four key amino acid residues of FGF-1 from the FGF-receptor binding site with alanines, constructing four point mutants and one double mutant. We determined by in vivo assays in NIH 3T3 cells the ability of the mutants to bind to specific FGF receptors, to stimulate DNA synthesis, and to activate downstream signaling pathways. We found that correct binding to the receptor is necessary for optimal stimulation of DNA synthesis. All four single mutants became phosphorylated to different extents, indicating that they were translocated to the cytosol/nucleus with varying efficiency. This indicates that despite a low affinity for FGFR, translocation to the cytosol/nucleus can still occur. However, simultaneous substitution in two of the positions led to a total loss of biological activity of the growth factor and prevented its internalization, implying that there is only one strongly receptor-dependent, productive way of translocating FGF-1. We also found that the process of translocation did not correlate with the thermal stability of the protein. Additionally, we observed a clear negative correlation between the stability of the FGF-1 mutants and the efficiency of their phosphorylation, which strongly suggests that protein kinases prefer the unfolded state of the protein substrate.

A nuclear export sequence located on a beta-strand in fibroblast growth factor-1

Receptor-bound and endocytosed fibroblast growth factor-1 (FGF-1) is able to cross the vesicle membrane and translocate to cytosol and nucleus. This suggests an intracellular role of FGF-1, which also signals by activating transmembrane FGF receptors. Phosphorylation of internalized FGF-1 by nuclear protein kinase C delta induces rapid export from the nuclei by a leptomycin B-sensitive pathway. In the present work, we have searched for and identified a Leu-rich nuclear export sequence (NES) at the C terminus of FGF-1 required for its nuclear export and able to confer nuclear export activity to a reporter protein in an in vivo system. Mutants where hydrophobic amino acids within the NES were exchanged for alanine exhibited reduced or abolished nuclear export. As demonstrated in co-immunoprecipitation experiments, a complex containing FGF-1, exportin-1, and its co-factor Ran-GTP, was formed in vitro. Formation of this complex in vivo was demonstrated by a peroxisomal targeting assay. Formation of the FGF-1-exportin-1-Ran-GTP complex in vitro as well as nuclear export of FGF-1 in vivo was dependent on phosphorylation of FGF-1, and it was abolished by leptomycin B. The FGF-1 NES was found to be situated along a beta-strand, which has not been reported before, since NESs usually are alpha-helical.

Fibroblast growth factor receptor-induced phosphorylation of STAT1 at the Golgi apparatus without translocation to the nucleus

STAT transcription factors signal from the plasma membrane to the nucleus in response to growth factors and cytokines, but little is known about activation of STAT1 from intracellular sites. Here we show that transient transfection of COS cells with fibroblast growth factor receptors (FGFRs) led to ligand-independent phosphorylation of the receptors, including intracellular immature forms. FGF-independent activation of STAT1 was demonstrated at the Golgi apparatus where it was colocalized with FGFRs. Both FGFR1 and FGFR2 induced strong phosphorylation of STAT1 causing redistribution of the Golgi apparatus, while FGFR3 and FGFR4 induced less phosphorylation of STAT1 and little or no redistribution of the Golgi apparatus. Upon expression of a cytosolic mutant of FGFR4 lacking the transmembrane as well as the extracellular region (CytR4), STAT1 was phosphorylated and transferred to the nucleus. The results indicate that immature forms of FGFRs form incomplete signaling complexes on Golgi membranes trapping phospho-STAT1 on this organelle.

Structural requirements for furin-induced cleavage and activation of Shiga toxin

Shiga toxin has a protease-sensitive site in the disulfide loop region of the A-chain. Cleavage of this site by furin is essential for rapid intoxication of cells by Shiga toxin. We have here investigated whether in addition to the Arg-X-X-Arg sequence, there are other structural requirements in the disulfide loop region for furin cleavage. A toxin mutant (Shiga-2D toxin) still containing the consensus motif for cleavage by furin, but lacking ten amino acids in the disulfide loop, was generated. Trypsin was able to cleave Shiga-2D toxin in vitro, demonstrating that the protease-sensitive region is intact. However, Shiga-2D toxin was not efficiently cleaved by furin either in vitro or in vivo. Furthermore, unless it was precleaved with trypsin, Shiga-2D toxin was much less toxic than wild type Shiga toxin in LoVo cells expressing functional furin. In contrast, LoVo/neo cells lacking functional furin were unable to activate both wild type Shiga toxin and Shiga-2D toxin. In conclusion, an extended loop structure is required for furin-induced cleavage of Shiga toxin.

Different abilities of the four FGFRs to mediate FGF-1 translocation are linked to differences in the receptor C-terminal tail

Members of the fibroblast growth factor family bind to one or more of the four closely related membrane-spanning FGF receptors. In addition to signaling through the receptors, exogenous FGF-1 and FGF-2 are endocytosed and translocated to the cytosol and nucleus where they stimulate RNA and DNA synthesis. Here we have studied the ability of the four FGF receptors to facilitate translocation of exogenous FGF-1 to the cytosol and nucleus. FGFR1 and FGFR4 were able to mediate translocation, whereas FGFR2 and FGFR3 completely lacked this ability. By analyzing mutant FGFRs we found that the tyrosine kinase domain could be deleted from FGFR1 without abolishing translocation, whereas the C-terminal tail of the FGFRs, constituted by approximately 50 amino acids downstream of the kinase domain, plays a crucial role in FGF-1 translocation. Three amino acids residues within the C-terminal tail were found to be of particular importance for translocation. For FGFR2, the two amino acid substitutions Q774M and P800H were sufficient to enable the receptor to support FGF-1 translocation. The results demonstrate a striking diversity in function of the four FGFRs determined by their C-terminal domain.

Reduced binding of FGF1 to mutant fibroblast growth factor receptor 3

The activating mutation FGFR3-R248C in the D2-D3 linker region of fibroblast growth factor receptor 3 leads as germline mutation to the neonatal lethal syndrome thanatophoric dysplasia type I (TD1). As somatic mutation it has been found in cancer. We introduced into the murine FGFR3 the mutation R242C that is orthologoues to the human mutation R248C. A strong reduction in binding of the 16 and 18 kDa forms of FGF1 to the mutant receptor was found, highlighting the importance of D2-D3 linker region of FGFR3 in determination of binding affinity to ligands. Another mutant, G374R, introduced into the murine FGFR3, is orthologoues to the human mutant FGFR3-G380R, and leads to achondroplasia (ACH). The binding of the 16 kDa and 18 kDa forms of FGF1 to this mutant receptor was the same as for wild-type FGFR3 in a cell-free system, but it was reduced in living cells. The data indicate a minor changes in conformation of FGFR3-G374R receptors at the cell surface that lead to reduced binding to FGF1.

FGF-1 and FGF-2 Require the Cytosolic Chaperone Hsp90 for Translocation into the Cytosol and the Cell Nucleus

Similarly to many protein toxins, the growth factors fibroblast growth factor 1 (FGF-1) and FGF-2 translocate from endosomes into the cytosol. It was recently found that certain toxins are dependent on cytosolic Hsp90 for efficient translocation across the endosomal membrane. We therefore investigated the requirement for Hsp90 in FGF translocation. We found that low concentrations of the specific Hsp90 inhibitors, geldanamycin and radicicol, completely blocked the translocation of FGF-1 and FGF-2 to the cytosol and the nucleus. The drugs did not interfere with the initial binding of FGF-1 to the growth factor receptors at the cell-surface or with the subsequent internalization of the growth factors into endosomes. The activation of known signaling cascades downstream of the growth factor receptors was also not affected by the drugs. The data indicate that the drugs block translocation from endosomes to the cytosol implying that Hsp90 is required for translocation of FGF-1 and FGF-2 across the endosomal membrane.

Highly stable mutants of human fibroblast growth factor-1 exhibit prolonged biological action

Fibroblast growth factor 1 (FGF-1) shows strong angiogenic, osteogenic and tissue-injury repair properties that might be relevant to medical applications. Since FGF-1 is partially unfolded at physiological temperature we decided to increase significantly its conformational stability and test how such an improvement will affect its biological function. Using an homology approach and rational strategy we designed two new single FGF-1 mutations: Q40P and S47I that appeared to be the most strongly stabilizing substitutions among those reported so far, increasing the denaturation temperature by 7.8 deg. C and 9.0 deg. C, respectively. As our goal was to produce highly stable variants of the growth factor, we combined these two mutations with five previously described stabilizing substitutions. The multiple mutants showed denaturation temperatures up to 27 deg. C higher than the wild-type and exhibited full additivity of the mutational effects. All those mutants were biologically competent in several cell culture assays, maintaining typical FGF-1 activities, such as binding to specific cell surface receptors and activation of downstream signaling pathways. Thus, we demonstrate that the low denaturation temperature of wild-type FGF-1 is not related to its fundamental cellular functions, and that FGF-1 action is not affected by its stability. A more detailed analysis of the biological behavior of stable FGF-1 mutants revealed that, compared with the wild-type, their mitogenic properties, as probed by the DNA synthesis assay, were significantly increased in the absence of heparin, and that their half-lives were extensively prolonged. We found that the biological action of the mutants was dictated by their susceptibility to proteases, which strongly correlated with the stability. Mutants which were much more resistant to proteolytic degradation always displayed a significant improvement in the half-life and mitogenesis. Our results show that engineered stable growth factor variants exhibit enhanced and prolonged activity, which can be advantageous in terms of the potential therapeutic applications of FGF-1.

[Ricin--a therapeutic agent and a biological weapon]

Lately there have been several incidents with the plant protein ricin, for instance in the capitol building of the American Senate in February 2004. By focusing on this toxin, the media have spread a fear that ricin will be used in terrorist acts. In this article, we want to give a review of the qualities and potentials of the use of ricin. Ricin is a highly toxic plant protein that is lethal in small doses; it kills cells by inactivating ribosomes but has potential for medical use.

Different intracellular trafficking of FGF1 endocytosed by the four homologous FGF receptors

Many growth factors and cytokines bind to more than one receptor, but in many cases the different roles of the separate receptors in signal transduction are unclear. Intracellular sorting of ligand-receptor complexes may modulate the signalling, and we have here studied the intracellular trafficking of ligand bound to receptors for fibroblast growth factors (FGFs). For this purpose, we transfected HeLa cells with any one of the four tyrosine kinase FGF receptors (FGFR1-4). In cells expressing any one of these receptors, externally added FGF1 was localized to sorting/early endosomes after 15 minutes at 37 degrees C. After longer incubation times, FGF1 internalized in cells expressing FGFR1 was localized mainly to late endosomes/lysosomes, similarly to EGF. By contrast, FGF1 internalized in cells expressing FGFR4 followed largely the same intracellular pathway as the recycling ligand, transferrin. In cells expressing FGFR2 or FGFR3, sorting of FGF1 to lysosomes was somewhat less efficient than that observed for FGFR1. Furthermore, FGF1 was more slowly degraded in cells expressing FGFR4 than in cells expressing FGFR1-3 and in addition, internalized FGFR4 as such was more slowly degraded than the other receptors. The data indicate that after endocytosis, FGFR4 and its bound ligand are sorted mainly to the recycling compartment, whereas FGFR1-3 with ligand are sorted mainly to degradation in the lysosomes. Alignment of the amino acid sequence of the intracellular part of the four FGFRs revealed several lysines conserved in FGFR1-3 but absent in FGFR4. Lysines are potential ubiquitylation sites and could thus target a receptor to lysosomes for degradation. Indeed, we found that FGFR4 is less ubiquitylated than FGFR1, which could be the reason for the different sorting of the receptors.

Two nuclear localization signals required for transport from the cytosol to the nucleus of externally added FGF-1 translocated into cells

Externally added FGF-1 is transported into the nucleus of cells. It was earlier shown that FGF-1 contains an N-terminal nuclear localization signal (NLS) implicated in the stimulation of DNA synthesis. We here provide evidence that FGF-1 contains a second putative NLS (NLS2), which is located near the C-terminus. It is a bipartite NLS consisting of two clusters of lysines separated by a spacer of 10 amino acids. A fusion protein of GFP and the bipartite NLS was more efficiently transported into the nucleus than GFP alone, indicating that it can act as an NLS in the living cell. FGF-1 mutated in the N-terminal NLS (NLS1) or in the first cluster of the bipartite NLS2 bound to heparin and FGF receptors and activated downstream signaling similarly to the wild-type growth factor. Mutations in the second cluster of NLS2 resulted in impaired interaction with heparin and reduced stability. When radiolabeled FGF-1 with mutated NLS1 or the first lysine cluster of NLS2 was added to NIH/3T3 cells, it was translocated into the cytosol, but not transported efficiently to the nucleus. Phosphorylation of FGF-1 occurs normally in the nucleus, and while wild-type FGF-1 was phosphorylated after addition to cells, the NLS mutants were not. It therefore appears that both NLS1 and NLS2 are important for efficient transport of FGF-1 to the nucleus. Stimulation of DNA synthesis by FGF-1 with mutations in both NLSs was reduced considerably indicating that efficient transport to the nucleus may be involved in the stimulation of DNA synthesis.

The Rab5 effector Rabaptin-5 and its isoform Rabaptin-5delta differ in their ability to interact with the small GTPase Rab4

Rabaptin-5 is an effector for the small GTPase Rab5, a regulator of the early steps in endocytosis. In addition, Rabaptin-5 interacts with the small GTPase Rab4 that has been implicated in recycling from early endosomes to the cell surface. Recently we have identified a ubiquitous transcript encoding the Rabaptin-5 isoform, Rabaptin-5delta. To evaluate the interaction properties of Rabaptin-5delta with the small GTPases Rab4 and Rab5, we have applied protein interaction assays using the yeast two-hybrid system and a glutathione S-transferase pull-down assay. We found that unlike Rabaptin-5, that interacts with both GTPases in GTP-bound conformations, Rabaptin-5delta interacts only with GTP-bound Rab5, and does not interact with Rab4, presumably due to a disrupted Rab4 binding site. Immunofluorescence microscopy analysis carried out to address the localization of Rabaptin-5delta relative to GTP-bound Rab4 and Rab5 in BHK-21 cells supported these data. Our data suggests that while Rabaptin-5 was proposed to act as a molecular linker between Rab5 and Rab4, to coordinate endocytic and recycling traffic, Rabaptin-5delta is involved only in the Rab5-driven events.

The Rab5 effector Rabaptin-5 and its isoform Rabaptin-5delta differ in their ability to interact with the small GTPase Rab4

Rabaptin-5 is an effector for the small GTPase Rab5, a regulator of the early steps in endocytosis. In addition, Rabaptin-5 interacts with the small GTPase Rab4 that has been implicated in recycling from early endosomes to the cell surface. Recently we have identified a ubiquitous transcript encoding the Rabaptin-5 isoform, Rabaptin-5delta. To evaluate the interaction properties of Rabaptin-5delta with the small GTPases Rab4 and Rab5, we have applied protein interaction assays using the yeast two-hybrid system and a glutathione S-transferase pull-down assay. We found that unlike Rabaptin-5, that interacts with both GTPases in GTP-bound conformations, Rabaptin-5delta interacts only with GTP-bound Rab5, and does not interact with Rab4, presumably due to a disrupted Rab4 binding site. Immunofluorescence microscopy analysis carried out to address the localization of Rabaptin-5delta relative to GTP-bound Rab4 and Rab5 in BHK-21 cells supported these data. Our data suggests that while Rabaptin-5 was proposed to act as a molecular linker between Rab5 and Rab4, to coordinate endocytic and recycling traffic, Rabaptin-5delta is involved only in the Rab5-driven events.

Phosphorylation-regulated nucleocytoplasmic trafficking of internalized fibroblast growth factor-1

Fibroblast growth factor-1 (FGF-1), which stimulates cell growth, differentiation, and migration, is capable of crossing cellular membranes to reach the cytosol and the nucleus in cells containing specific FGF receptors. The cell entry process can be monitored by phosphorylation of the translocated FGF-1. We present evidence that phosphorylation of FGF-1 occurs in the nucleus by protein kinase C (PKC)delta. The phosphorylated FGF-1 is subsequently exported to the cytosol. A mutant growth factor where serine at the phosphorylation site is exchanged with glutamic acid, to mimic phosphorylated FGF-1, is constitutively transported to the cytosol, whereas a mutant containing alanine at this site remains in the nucleus. The export can be blocked by leptomycin B, indicating active and receptor-mediated nuclear export of FGF-1. Thapsigargin, but not leptomycin B, prevents the appearance of active PKCdelta in the nucleus, and FGF-1 is in this case phosphorylated in the cytosol. Leptomycin B increases the amount of phosphorylated FGF-1 in the cells by preventing dephosphorylation of the growth factor, which seems to occur more rapidly in the cytoplasm than in the nucleus. The nucleocytoplasmic trafficking of the phosphorylated growth factor is likely to play a role in the activity of internalized FGF-1.

The history of ricin, abrin and related toxins

Ricin, abrin and related plant toxins have played interesting and important roles in the history of clinical medicine and biomedical research. The use of these proteins in medical treatment since ancient times is reviewed. Later the proteins played important roles in the early days of immunological research and some of the fundamental principles of immunology were discovered with toxic proteins of this group. During the last three decades the mechanism of action of the toxins was elucidated. This led to a major effort to target the toxins to malignant cells. Ricin has been used in bioterrorism. Recently, the toxins have played important roles as experimental models to elucidate the intracellular trafficking of endocytosed proteins.

Trafficking and signaling pathways of nuclear localizing protein ligands and their receptors

Interaction of ligands such as epidermal growth factor and interferon-gamma with the extracellular domains of their plasma membrane receptors results in internalization followed by translocation into the nucleus of the ligand and/or receptor. There has been reluctance, however, to ascribe signaling importance to this, the focus instead being on second messenger pathways, including mobilization of kinases and inducible transcription factors (TFs). The latter, however, fails to explain the fact that so many ligands stimulate the same second messenger cascades/TFs, and yet show distinct gene activation profiles. This is particularly apt in the case of the seven STAT TFs that are held to be the mediators of the distinct cellular functions of over 60 ligands. The current review focuses on five representative nuclear localizing ligands for which there is documentation of translocation into the cytosol and nucleus through well-characterized pathways, in addition to a role in gene activation by ligand/receptor in the nucleus.

Cloning and characterization of the genes encoding toxic lectins in mistletoe (Viscum album L)

Leaves of mistletoe (Viscum album L) contain three toxic lectins (type 2 ribosome-inactivating proteins) MLI, MLII, and MLIII, differing in molecular mass and carbohydrate specificity. Clones, containing sequences of three gene variants designated ml1p, ml2p, and ml3p, were obtained using PCR amplification from cDNA and from mistletoe genomic DNA. The quantitative ratio of the ml1p, ml2p, and ml3p genes in genomic DNA was found to be 1.5 : 1 : 4, respectively, whereas the ratio of their mRNA was 50 : 10 : 1. The quantitative prevalence of the ml1p transcript correlates well with the observation that MLI is quantitatively dominant over MLII and MLIII in the mistletoe extract. The sequences of the proteins encoded by the ml1p, ml2p, and ml3p genes are identical to MLI by 98, 88, and 77%, respectively. The similarity to MLI of the amino acid sequence encoded by the gene ml1p, the quantitative prevalent of its mRNA, as well as structural properties of the B-chain indicate that the gene, ml1p, corresponds to MLI. Western blot analysis of recombinant A-chains encoded by the three variants of mlp genes with the monoclonal antibody MNA4 having differential affinity to MLI, MLII and MLIII A-chains suggests that the ml2p and ml3p genes correspond to MLII and MLIII, respectively. Structural differences in the carbohydrate-binding sites of the B-subunits of ML1p, ML2p, and ML3p probably explain the difference in sugar specificity of MLI, MLII and MLIII.

Deletion mutant of FGFR4 induces onion-like membrane structures in the nucleus

The expression of several deletion mutants of fibroblast growth factor receptor 4 (FGFR4) was studied in COS-1 cells. FGFR4-mutants lacking most of the extracellular region did not efficiently reach the plasma membrane but accumulated in the endoplasmic reticulum (ER) and Golgi body. A mutant FGFR4 lacking the kinase domain as well as most of the extracellular region (DeltaExt/R4Tth) had a distinct intracellular distribution. It localized in part to the nucleus, where it exhibited a striking spotted pattern. Ultrastructural studies showed that the nuclear spots consisted of several layers of membrane that were folded into onion-like structures at the nucleoplasmic side of the nuclear envelope. These intranuclear structures did not contain nuclear pores but were positive for the ER proteins calreticulin and protein disulfide isomerase, in addition to abundant DeltaExt/R4Tth. Formation of the intranuclear structures was sensitive to inhibition of protein kinase C. Live microscopy of a green-fluorescent-protein/DeltaExt/R4Tth fusion protein showed that the intranuclear structures were stable and immobile, suggesting that they function as deposits of the overexpressed mutant and associated membrane. The DeltaExt/R4Tth protein also induced formation of densely packed membrane stacks in the cytosol and we suggest a model were the intranuclear structures are formed by invagination of ER-derived membrane stacks into the nucleus.

Peroxisomal Targeting as a Tool for Assaying Protein-Protein Interactions in the Living Cell

Current methods to detect protein-protein interactions are either laborious to implement or not adaptable for mammalian systems or in vitro methods. By adding a peroxisomal targeting signal (PTS) onto one protein, binding partners lacking a targeting signal were co-transported into the peroxisomes in a "piggy-back" fashion, as visualized by confocal and electron microscopy. A fragment of colicin E2 and its tightly interacting immunity protein, ImmE2, were both expressed in the cytosol. When either one contained a PTS tag, both proteins were co-localized in the peroxisomes. The cytokine-independent survival kinase (CISK) containing a PTS tag was not efficiently targeted to the peroxisomes unless the Phox homology (PX) domain, attaching the protein to endosomal membranes, was removed. However, PTS-tagged CISK with deleted PX domain was able to direct 3-phosphoinositide-dependent protein kinase-1 (PDK-1) into the peroxisomes. This demonstrates that the two proteins interact in vivo. Mutating Ser486, which is phosphorylated in activated CISK, to Ala prevented the interaction, indicating that CISK and PDK-1 interact in a phosphorylation-dependent manner. The method therefore allows assessment of protein-protein interactions that depend on post-translational modifications that are cell-specific or dependent on the physiological state of the cell.

Translocation of FGF-1 and FGF-2 across vesicular membranes occurs during G1-phase by a common mechanism

The entry of exogenous fibroblast growth factor 2 (FGF-2) to the cytosolic/nuclear compartment was studied and compared with the translocation mechanism used by FGF-1. To differentiate between external and endogenous growth factor, we used FGF-2 modified to contain a farnesylation signal, a CaaX-box. Because farnesylation occurs only in the cytosol and nucleoplasm, farnesylation of exogenous FGF-2-CaaX was taken as evidence that the growth factor had translocated across cellular membranes. We found that FGF-2 translocation occurred in endothelial cells and fibroblasts, which express FGF receptors, and that the efficiency of translocation was increased in the presence of heparin. Concomitantly with translocation, the 18-kDa FGF-2 was N-terminally cleaved to yield a 16-kDa form. Translocation of FGF-2 required PI3-kinase activity but not transport through the Golgi apparatus. Inhibition of endosomal acidification did not prevent translocation, whereas dissipation of the vesicular membrane potential completely blocked it. The data indicate that translocation occurs from intracellular vesicles containing proton pumps and that an electrical potential across the vesicle membrane is required. Translocation of both FGF-1 and FGF-2 occurred during most of G(1) but decreased shortly before the G(1)-->S transition. A common mechanism for FGF-1 and FGF-2 translocation into cells is postulated.

Characterization and tissue expression of acidic fibroblast growth factor binding protein homologue in Drosophila melanogaster

We have earlier reported a Drosophila protein, which aligned significantly with the amino acid sequence of the human acidic fibroblast growth factor intracellular binding protein (FIBP). In attempts to further elucidate the function of FIBP and its putative role in fibroblast growth factor (FGF) signaling we have cloned and characterized FIBP from Drosophila melanogaster (DrFIBP). Using comparative sequence analysis of Drosophila and human FIBP genes we demonstrate a remarkable conservation of their structural architecture suggesting that FIBP from vertebrates and insects are genuine homologues. Reverse transcriptase polymerase chain reaction analysis of FIBP mRNA from Drosophila revealed differential splicing by intron retention resulting in the production of three distinct FIBP transcripts. The retention of the intronic sequences introduces termination codons within the mature FIBP mRNA leading to premature termination of translation. Analysis of FIBP mRNA distribution in the fruit fly suggests that DrFIBP, like its mammalian homologue, is an abundant protein whose expression is maintained during embryonic, larval and adult stages. The spatial expression pattern investigated by whole mount embryo immunostaining reveals expression of FIBP in the developing tracheal system and in ventral midline cells, two known sites of FGF signaling in the fruit fly.

Transport of exogenous growth factors and cytokines to the cytosol and to the nucleus

In recent years a number of growth factors, cytokines, protein hormones, and other proteins have been found in the nucleus after having been added externally to cells. This review evaluates the evidence that translocation takes place and discusses possible mechanisms. As a demonstration of the principle that extracellular proteins can penetrate cellular membranes and reach the cytosol, a brief overview of the penetration mechanism of protein toxins with intracellular sites of action is given. Then problems and pitfalls in attempts to demonstrate the presence of proteins in the cytosol and in the nucleus as opposed to intracellular vesicular compartments are discussed, and some new approaches to study this are described. A detailed overview of the evidence for translocation of fibroblast growth factor, HIV-Tat, interferon-gamma, and other proteins where there is evidence for intracellular action is given, and translocation mechanisms are discussed. It is concluded that although there are many pitfalls, the bulk of the experiments indicate that certain proteins are indeed able to enter the cytosol and nucleus. Possible roles of the internalized proteins are discussed.

Vesicle transmembrane potential is required for translocation to the cytosol of externally added FGF-1

Externally added fibroblast growth factor-1 (FGF-1) is capable of crossing cellular membranes to reach the cytosol and the nucleus in a number of cell types. We have monitored the translocation of the growth factor by two methods: phosphorylation of FGF-1, and prenylation of an FGF-1 mutant that contains a C-terminal prenylation signal. Inhibition of endosomal acidification by ammonium chloride or monensin did not block the translocation of FGF-1, whereas bafilomycin A1, a specific inhibitor of vacuolar proton pumps, blocked translocation completely. A combination of ionophores expected to dissipate the vesicular membrane potential (valinomycin plus monensin) also fully inhibited the translocation. The inhibition of translocation by bafilomycin A1 was overcome in the presence of monensin or nigericin, while ouabain blocked translocation under these conditions. The data indicate that translocation of FGF-1 to cytosol occurs from the lumen of intracellular vesicles possessing vacuolar proton pumps, and that a vesicular membrane potential is required. Apparently, activation of vesicular Na+/K+-ATPase by monensin or nigericin generates a membrane potential that can support translocation when the proton pump is blocked.

Binding of FGF-1 variants to protein kinase CK2 correlates with mitogenicity

Fibroblast growth factor-1 (FGF-1) has both extra- and intracellular functions. To identify intracellular binding partners for FGF-1, we isolated proteins from U2OS human osteosarcoma cells interacting specifically with FGF-1. One of the isolated proteins was identified as protein kinase CK2 (CK2). We here provide evidence that FGF-1 binds to both the catalytic alpha-subunit and to the regulatory beta-subunit of CK2. The interaction between FGF-1 and CK2 alpha and beta was characterized by surface plasmon resonance, giving K(D) values of 0.4 +/- 0.3 and 1.2 +/- 0.2 microM, respectively. By using a novel assay for intracellular protein interaction, FGF-1 and CK2 alpha are shown to interact in vivo. In vitro, FGF-1 and FGF-2 are phosphorylated by CK2, and the presence of FGF-1 or FGF-2 was found to enhance the autophosphorylation of CK2 beta. A correlation between the mitogenic potential of FGF-1 mutants and their ability to bind to CK2 alpha was observed. The possible involvement of CK2 in the FGF-induced stimulation of DNA synthesis is discussed.

Identification of ribosome-binding protein p34 as an intracellular protein that binds acidic fibroblast growth factor

With the aim of identifying new intracellular binding partners for acidic fibroblast growth factor (aFGF), proteins from U2OS human osteosarcoma cells were adsorbed to immobilized aFGF. One of the adsorbed proteins is a member of the leucine-rich repeat protein family termed ribosome-binding protein p34 (p34). This protein has previously been localized to endoplasmic reticulum membranes and is thought to span the membrane with the N terminus on the cytosolic side. Confocal microscopy of cells transfected with Myc-p34 confirmed the endoplasmic reticulum localization, and Northern blotting determined p34 mRNA to be present in a multitude of different tissues. Cross-linking experiments indicated that the protein is present in the cell as a dimer. In vitro translated p34 was found to interact with maltose-binding protein-aFGF through its cytosolic coiled-coil domain. The interaction between aFGF and p34 was further characterized by surface plasmon resonance, giving a K(D) of 1.4 +/- 0.3 microm. Even though p34 interacted with mitogenic aFGF, it bound poorly to the non-mitogenic aFGF(K132E) mutant, indicating a possible involvement of p34 in intracellular signaling by aFGF.

[A short form of the heparin-binding EGF-like growth factor with a changed EGF domain]

In all secreted proteins related to the epidermal growth factor (EGF), EGF domains that occur in a mature factor are each encoded by two exons, and those that do not, by one exon. During splicing, additional exon 3a can be inserted between exons 3 and 4, which code for the EGF domain of the mature heparin-binding EGF-like growth factor (HB-EGF). The resulting mRNA codes for the short form of HB-EGF (SF HB-EGF), which retains the signal peptide, the propeptide, and the heparin-binding domain. However, its EGF domain lacks the C-terminal subdomain essential for the interaction with the EGF receptor (EGFR). Structural analysis suggested that SF HB-EGF is a secreted polypeptide that has high affinity for heparin, but weakly, if at all, interacts with EGFR. Data obtained in three different systems indicated that SF HB-EGF possesses a mitogenic activity but utilizes a signal transduction pathway other than that of HB-EGF.

Ricin

The plant toxin ricin consists of two disulfide-linked polypeptides with different functions. The A-chain enters the cytosol and inactivates the ribosomes enzymatically, whereas the B-chain has lectin properties and binds to carbohydrates at the cell surface. This binding is a requirement for translocation of the A-chain to the cytosol. The bound toxin is endocytosed and transported retrograde through the Golgi apparatus to the endoplasmic reticulum where it appears to be translocated to the cytosol by the sec61p complex.

Modulation of intracellular transport of acidic fibroblast growth factor by mutations in the cytoplasmic receptor domain

Endocytic uptake and intracellular transport of acidic fibroblast growth factor (aFGF) was studied in cells transfected with FGF receptor 4 with mutations in the cytoplasmic part. Endocytic uptake in HeLa cells was reduced but not abolished when the tyrosine kinase of the receptor was inactivated by mutations or deletions. The tyrosine kinase-dependent endocytosis of aFGF was prevented by the expression of a dominant negative dynamin mutant that blocks endocytosis from coated pits and caveolae. However, more than half of the total endocytic uptake of aFGF was not affected under these conditions, indicating an endocytic uptake mechanism not involving coated pits or caveolae. Mutation or deletion of a putative caveolin-binding sequence did not prevent the localization of part of the receptors to a low density, caveolin-containing subcellular fraction. Whereas wild-type receptor transfers the growth factor from early endosomes to the recycling compartment, kinase negative, full length receptors were inefficient in this respect and the growth factor instead accumulated in lysosomes. By contrast, when most of the intracellular part of the receptor, including the kinase domain, was removed, aFGF was transported to the recycling compartment, as in cells that express wild-type receptors, suggesting the presence of a kinase-regulated targeting signal in the cytoplasmic tail.

Ability of the Tat basic domain and VP22 to mediate cell binding, but not membrane translocation of the diphtheria toxin A-fragment

A number of proteins are able to enter cells from the extracellular environment, including protein toxins, growth factors, viral proteins, homeoproteins, and others. Many such translocating proteins, or parts of them, appear to be able to carry with them cargo into the cell, and a basic sequence from the HIV-1 Tat protein has been reported to provide intracellular delivery of several fused proteins. For evaluating the efficiency of translocation to the cytosol, this TAT-peptide was fused to the diphtheria toxin A-fragment (dtA), an extremely potent inhibitor of protein synthesis which normally is delivered efficiently to the cytosol by the toxin B-fragment. The fusion of the TAT-peptide to dtA converted the protein to a heparin-binding protein that bound avidly to the cell surface. However, no cytotoxicity of this protein was detected, indicating that the TAT-peptide is unable to efficiently deliver enzymatically active dtA to the cytosol. Interestingly, the fused TAT-peptide potentiated the binding and cytotoxic effect of the corresponding holotoxin. We made a fusion protein between VP22, another membrane-permeant protein, and dtA, and also in this case we detected association with cells in the absence of a cytotoxic effect. The data indicate that transport of dtA into the cell by the TAT-peptide and VP22 is inefficient.

An interaction between ricin and calreticulin that may have implications for toxin trafficking

Here we demonstrate that ricin is able to interact with the molecular chaperone calreticulin both in vitro and in vivo. The interaction occurred with ricin holotoxin, but not with free ricin A chain; and it was prevented in the presence of lactose, suggesting that it was mediated by the lectin activity of the ricin B chain. This lectin is galactose-specific, and metabolic labeling with [(3)H]galactose or treating galactose oxidase-modified calreticulin with sodium [(3)H]borohydride indicated that Vero cell calreticulin possesses a terminally galactosylated oligosaccharide. Brefeldin A treatment indicated that the intracellular interaction occurred initially in a post-Golgi stack compartment, possibly the trans-Golgi network, whereas the reductive separation of ricin subunits occurred in an earlier part of the secretory pathway, most probably the endoplasmic reticulum (ER). Intoxicating Vero cells with ricin whose A chain had been modified to include either a tyrosine sulfation site or the sulfation site plus available N-glycosylation sites, in the presence of Na(2)35SO(4), confirmed that calreticulin interacted with endocytosed ricin that had already undergone retrograde transport to both the Golgi and the ER. Although we cannot exclude the possibility that the interaction between ricin and calreticulin is an indirect one, the data presented are consistent with the idea that calreticulin may function as a recycling carrier for retrograde transport of ricin from the Golgi to the ER.

[Two forms of precursors of a heparin-binding EGF-like growth factor-diphtheria toxin receptor]

A new mRNA coding for the heparin-binding EGF-like growth factor (HB-EGF) was found in Vero cells. The corresponding cDNA had C-156 in place of T, which resulted in a loss of the NheI site and a substitution of Leu-33 with Pro in the HB-EGF precursor. The known and new forms of the precursor were accordingly termed L and P. A possible conformational change in the corresponding propeptide region were assumed to affect processing of soluble secreted HB-EGF. The L and P mRNAs are differently expressed in various cell lines and have the identical 5'-untranslated sequences. Possibly, they are transcribed from one promoter and then alternatively spliced. Stimulation of resting Vero cells with tetraphorbol ester (TPA) substantially increased production of the L form, decreased production of the P form, and did not affect expression of the total HB-EGF mRNA. This was associated with an increase in binding of the diphtheria toxin, suggesting that the L HB-EGF precursor acts as its receptor.

Organization, chromosomal localization and promoter analysis of the gene encoding human acidic fibroblast growth factor intracellular binding protein

Acidic fibroblast growth factor (aFGF) intracellular binding protein (FIBP) is a protein found mainly in the nucleus that might be involved in the intracellular function of aFGF. Here we present a comparative analysis of the deduced amino acid sequences of human, murine and Drosophila FIBP analogues and demonstrate that FIBP is an evolutionarily conserved protein. The human gene spans more than 5 kb, comprising ten exons and nine introns, and maps to chromosome 11q13.1. Two slightly different splice variants found in different tissues were isolated and characterized. Sequence analysis of the region surrounding the translation start revealed a CpG island, a classical feature of widely expressed genes. Functional studies of the promoter region with a luciferase reporter system suggested a strong transcriptional activity residing within 600 bp of the 5' flanking region.

Externally added aFGF mutants do not require extensive unfolding for transport to the cytosol and the nucleus in NIH/3T3 cells

Acidic fibroblast growth factor (aFGF) is transported to the cytosol and the nucleus when added to cells expressing FGF receptors, implying that aFGF must cross cellular membranes. Since protein translocation across membranes commonly requires extensive unfolding of the protein, we were interested in testing whether this is also necessary for membrane translocation of aFGF. We therefore constructed mutant growth factors with intramolecular disulfide bonds to prevent complete unfolding. Control experiments demonstrated that translocation of aFGF by the diphtheria toxin pathway, which requires extensive unfolding of the protein, was prevented by disulfide bond formation, indicating that the introduced disulfide bonds interfered with the unfolding of the growth factor. On the other hand, when the growth factor as such was added to cells expressing FGF receptors, the disulfide-bonded mutants were translocated to the cytosol and the nucleus equally well as wild-type aFGF. The possibility that the translocation of the mutants was due to reduction of the disulfide bonds prior to translocation was tested in experiments using an irreversibly cross-linked mutant. Also this mutant was transported to the cytosol and to the nucleus. The results suggest that extensive unfolding is not required for membrane translocation of aFGF.

Requirement for C-terminal end of fibroblast growth factor receptor 4 in translocation of acidic fibroblast growth factor to cytosol and nucleus

The ability of COS cells to bind and internalise acidic fibroblast growth factor (aFGF) was studied after transient transfection of the cells with wild-type and mutated fibroblast growth factor receptor 4. In one case the tyrosine kinase of the receptor was inactivated by a point mutation in the active site, whereas in other cases parts of the receptor were deleted to remove various parts of the cytoplasmic domain. In all cases the receptors were expressed at the cell surface at a high level and the cells bound labelled growth factor efficiently and internalised it by endocytosis. Translocation of externally added aFGF across cellular membranes to reach the cytosol and nucleus was measured as transport of labelled growth factor to the nuclear fraction obtained by centrifugation, by farnesylation of growth factor modified to carry a CAAX motif, and by phosphorylation of the growth factor at a site specific for protein kinase C. Whereas both full-length receptors (with and without an active kinase domain) facilitated translocation of the growth factor to the cytosol and nucleus, as assessed by these methods, the mutants of the receptor where the C terminus was deleted, were unable to do so. In contrast, a receptor containing only the 57 most C-terminal amino acids of the cytoplasmic domain in addition to the juxtamembrane, transmembrane and extracellular domains, was in fact able to mediate translocation of aFGF to the cytosol. These data indicate that information contained in the C terminus of the receptor is required for translocation.

Requirement of phosphatidylinositol 3-kinase activity for translocation of exogenous aFGF to the cytosol and nucleus

Acidic fibroblast growth factor (aFGF) is a potent mitogen for many cells. Exogenous aFGF is able to enter the cytosol and nucleus of sensitive cells. There are indications that both activation of the receptor tyrosine kinase and translocation of aFGF to the nucleus are of importance for mitogenesis. However, the mechanism of transport of aFGF from the cell surface to the nucleus is poorly understood. In this work we demonstrate that inhibition of phosphatidylinositol (PI) 3-kinase by chemical inhibitors and by expression of a dominant negative mutant of PI 3-kinase blocks translocation of aFGF to the cytosol and nucleus. Translocation to the cytosol and nucleus was monitored by cell fractionation, by farnesylation of aFGF modified to contain a farnesylation signal, and by phosphorylation by protein kinase C of aFGF added externally to cells. If aFGF is fused to diphtheria toxin A-fragment, it can be artificially translocated from the cell surface to the cytoplasm by the diphtheria toxin pathway. Upon further incubation, the fusion protein enters the nucleus due to a nuclear localization sequence in aFGF. We demonstrate here that upon inhibition of PI 3-kinase the fusion protein remains in the cytosol. We also provide evidence that the phosphorylation status of the fusion protein does not regulate its nucleocytoplasmic distribution.

Requirement for prolonged action in the cytosol for optimal protein synthesis inhibition by diphtheria toxin

Diphtheria toxin A-fragment enters the cytosol of target cells, where it inhibits protein synthesis by catalyzing ADP-ribosylation of elongation factor 2 (EF-2). We have here analyzed toxin-induced protein synthesis inhibition in single cells by autoradiography and compared it with inhibition of protein synthesis in the whole cell culture. The data show that half-maximal protein synthesis inhibition in the whole cell population after a short incubation time is achieved by partially inhibiting protein synthesis in basically all the cells, while half-maximal protein synthesis inhibition after a long incubation time is due to a complete protein synthesis block in about half the cells in the population. We have also compared stable and unstable A-fragment mutants with respect to the kinetics of cell intoxication. While the toxicity of the stable mutants increased with time, the unstable mutants showed a similar toxicity at early and late time points. When studying the kinetics of cell intoxication by toxins with short cytosolic half-life, we could not detect any recovery of protein synthesis at late time points when all the mutant A-fragments should be degraded. This indicates that the ADP-ribosylation of EF-2 cannot be reversed by an endogenous activity in the cells. The data indicate that entry of toxin into a cell is not associated with an immediate block in protein synthesis, and that prolonged action of single A-fragment molecules in the cytosol is sufficient to obtain complete protein synthesis inhibition at low toxin concentrations.