Characterization of a novel polyclonal anti-hypusine antibody
© Nishiki et al.; licensee Springer. 2013
Received: 1 July 2013
Accepted: 26 August 2013
Published: 29 August 2013
The translation factor eIF5A is the only protein known to contain the amino acid hypusine, which is formed posttranslationally. Hypusinated eIF5A is necessary for cellular proliferation and responses to extracellular stressors, and has been proposed as a target for pharmacologic therapy. Here, we provide the first comprehensive characterization of a novel polyclonal antibody (IU-88) that specifically recognizes the hypusinated eIF5A. IU-88 will be useful for the investigation of eIF5A biology and for the development of assays recognizing hypusinated eIF5A.
Eukaryotic translation initiation factor 5A-1 and 5A-2 (eIF5A-1 and eIF5A-2—collectively referred to here as eIF5A) are highly conserved proteins whose varied cellular functions include the binding of and nucleocytoplasmic shuttling of specific mRNAs (Kruse et al. 2000; Maier et al. 2010; Xu and Chen 2001), cellular proliferation (Nishimura et al. 2005), and posttranslational stress responses (Li et al. 2010; Moore et al. 2008; Nishiki et al. 2013). Curiously, eIF5A is the only protein containing the amino acid hypusine, which is formed from a lysine residue in a posttranslational reaction involving the enzymes deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DHH) and the substrate spermidine (Park et al. 2010). In the complete absence of deoxyhypusine synthase mouse embryos die at a very early stage of development (Nishimura et al. 2012; Templin et al. 2011). Inhibition of hypusine formation has been suggested to confer cellular survival in certain stress states, such as infections, carcinogenesis, and obesity (Balabanov et al. 2007; Hauber et al. 2005; Robbins et al. 2010; Schwentke et al. 2012). Therefore, identification of the hypusinated form of eIF5A is paramount in the understanding of the biology of the protein. Nevertheless, identification of hypusinated eIF5A has remained a challenge, requiring tedious methods such as isoelectric focusing or two-dimensional gel electrophoresis of cellular extracts. Although prior studies reported the development of antibodies against hypusinated eIF5A (Bergeron et al. 1998; Cracchiolo et al. 2004), their characterizations were limited and utilities of these reagents were not described in subsequent reports. Here, we present the characterization of a novel anti-hypusine antibody reagent, IU-88. We demonstrate that IU-88 selectively recognizes either the deoxyhypusine or hypusine forms of eIF5A in vitro, and that IU-88 specifically recognizes the hypusinated form of eIF5A in cellular extracts by immunoblots and in whole cells by immunocytochemistry.
Materials and methods
Cell culture, transfection and DHS inhibition
Human 293T and rat INS-1(832/13) β cells were cultured as described (Hohmeier et al. 2000). Cells were transiently transfected with plasmids encoding EGFP-eIF5A, EGFP-eIF5A(K50A) and EGFP-DHS constructs using Lipofectamine 2000 (Invitrogen) for 16 hours before cell extraction or immunofluorescence analysis. The DHS inhibitor GC7 (Biosearch Technologies) was prepared and used in cell culture as previously described (Maier et al. 2010).
Reactions in vitro
For in vitro experiments, eIF5A protein was purified from E. coli as a GST fusion, after which the GST tag was proteolytically removed. DHS protein was purified from E. coli as an N-terminal His6 fusion. Purified human DHH protein was purchased from OriGene. The hypusination reactions in vitro proceeded as previously published (Wolff et al. 2011).
Antibodies and immunoblotting
The rabbit polyclonal antibody IU-88 against hypusinated human eIF5A was generated in rabbits using the synthetic hypusine-containing peptide C-Ahx-STSKTG[hypusine]HGHAKV-amide by contract to 21st Century Biochemicals. Monoclonal mouse pan-anti-eIF5A antibody was from BD Biosciences and anti-actin antibody was from MP Biomedicals. Immunoblot analysis was visualized using a LiCor Odyssey fluorescence system following electrophoresis on a 4-20% SDS polyacrylamide gel (Maier et al. 2010). Primary antibodies were diluted 1:1500 (IU-88) and 1:10,000 (anti-pan-eIF5A).
293T cells were fixed in 4% paraformaldehyde and immunocytochemistry proceeded as previously described (Robbins et al. 2010). Antibody dilutions were 1:150 for IU-88 and 1:1000 for anti-pan-eIF5A. 4',6-diamidino-2-phenylindole (DAPI) staining was used to visualize nuclei. A Zeiss LSM-710 microscope was used to visualize cells at magnification x100.
Results and discussion
Next, we tested the ability of IU-88 to specifically recognize hypusinated eIF5A in whole cellular extract by immunoblotting. As shown in the full gel image in Figure 1C, when extracts from rat-derived INS-1 islet β cells are used in immunoblotting, IU-88 recognizes only a single protein species at ~17 kDa, corresponding to the known molecular weight of eIF5A. Transfection of a plasmid encoding either a human EGFP-eIF5A(K50A) fusion protein (which is not capable of being hypusinated) or a human EGFP-eIF5A fusion protein results in the appearance of a protein species at ~44 kDa only with the EGFP-eIF5A transfection (Figure 1C, compare lanes 1 and 2). When these transfections are immunoblotted using an antibody against GFP, both EGFP-eIF5A(K50A) and EGFP-eIF5A are recognized (Figure 1D). These data demonstrate specificity of IU-88 in recognizing only eIF5A in total cellular protein, and also suggest that IU-88 only recognizes transfected eIF5A proteins that have the capability to be hypusinated. To investigate in greater detail the utility of IU-88 to distinguish hypusination in cellular extracts, we performed additional studies in human-derived 293T cells and rat-derived INS-1 cells. As shown in Figure 1E, when human 293T cells are transfected with GFP-eIF5A, a weak but detectable signal corresponding to GFP-eIF5A is observed using IU-88 (lane 1). This signal decreases further upon co-incubation with increasing concentrations of GC7 (Figure 1E, lanes 2 and 3), suggesting that IU-88 is recognizing the hypusine-specific form. Interestingly, when exogenous DHS is introduced by co-transfection of a GFP-DHS fusion protein-encoding vector, there is a dramatic increase in GFP-eIF5A signal as detected by IU-88 (Figure 1E, lane 4) with corresponding decrease in the presence of GC7 (lanes 5 and 6), suggesting that DHS protein levels may be limiting in the ability of 293T cells to hypusinate eIF5A—a finding that is also observed in human-derived HeLa cells (Lee et al. 2009). INS-1 β cells, by contrast, reveal a significantly different picture. As shown in Figure 1F, transfection of a plasmid encoding GFP-DHS did not enhance the signal observed with either GFP-eIF5A or endogenous eIF5A, suggesting that DHS is not limiting in the ability of INS-1 cells to hypusinate eIF5A. Interestingly, whereas increasing GC7 concentrations reduce the GFP-eIF5A signal observed with IU-88, it also reduces the signal observed with the pan-anti-eIF5A antibody (Figure 1F). This result suggests that INS-1 β cells may be unique in their requirement for hypusination to maintain production of eIF5A itself.
Figure 1E and F show that GC7 incubation also reduces, but only slightly, the level of endogenous eIF5A, as detected by IU-88 (and perhaps more-so in INS-1 cells than in 293T cells). Because IU-88 measures only steady-state levels of hypusinated eIF5A (as opposed to rate of hypusine formation—as measured by 3H-spermidine uptake studies, ref. (Maier et al. 2010)), this observation may reflect the long half-life (6–24 h) of the hypusinated eIF5A protein in mammalian cells (Gerner et al. 1986; Maier et al. 2010). More effective depletion of the hypusinated eIF5A species with GC7 may require longer periods of incubation.
Taken together, our results verify the specificity and utility of IU-88 in detecting a specifically modified form of eIF5A. Depending upon the application, an important caveat to the use of IU-88 is its inability to distinguish between the deoxyhypusinated and hypusinated forms of eIF5A. Although the relative significance of the deoxyhypusinated vs. hypusinated forms of eIF5A remains unclear, the low substrate Km of DHH relative to DHS means that the majority of eIF5A in cells is likely present in the fully hypusinated form (Park et al. 2010). Nevertheless, IU-88 represents an especially useful reagent for the assessment of at least the activity of DHS in cells. Also, because most pharmacologic approaches to inhibiting the hypusination reaction have focused on inhibition of the higher Km enzyme DHS, IU-88 would also serve as an important reagent for assessing DHS activity in drug screening studies.
Availability and requirements
This work was supported by grants R01 DK060581 (to RGM) and T32 DK064466 (to YN) from the National Institutes of Health and by a grant from the Juvenile Diabetes Research Foundation (to RGM).
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