The Biologic And Spectroscopic Studies On The N-Terminal Deletants Of Human Survivin In Vitro

Survivin is selectively expressed in the G2/M phase of the cell cycle and overexpressed in most human neoplasms, embryos and malignant tissues but not in normal tissues. It is the smallest member of the inhibitor of apoptosis protein (IAP) family and has dual functions in suppressing apoptosis and playing a central role in cell division. Detection of Survivin has prognostic value for some tumor cells and appears to be involved in their resistance to anticancer agents and ionizing radiation. Thus, all these properties render this protein an attractive target for cancer therapy.The Survivin monomer is a 16.5 kDa protein of 142 amino acids. Structurally, it consists of an N-terminal single globular baculovirusⅠAP repeat (BIR) domain (M1-S88), a linker segment (V89-T97), and a long amphipathic C-terminal coiled-coilα-helix (L98-D142). The Survivin BIR domain contains a zinc-binding fold similar to that found in the X-linkedⅠAP (ⅪAP) BIR2 and BIR3 domains, including three Cys and a His that bind a zinc ion, but it lacks the RING finger motif found inⅪAP. The secondary structure of the Survivin BIR domain consists of a three-strandedβ-sheet and fourα-helices. Both X-ray crystal diffraction and NMR spectroscopy of Survivin in solution have confirmed that it exists in two forms which confer different functions in vivo. Survivin participates in cell division as a monomer, associating with Borealin and inner centromere protein (INCENP) to form a complex called the chromosomal passenger complex (CPC) which interacts with Aurora-B kinase as a key regulator of chromosome segregation and cytokinesis during the cell cycle. On the other hand, the bow-tie-shaped homodimer form of Survivin has anti-apoptotic functions. The dimer interface is extensive, involving residues 6-13 in the N-terminal portion and a 14-amino acid region encompassing residues 89-102 located just in and after the linker segment. Hydrophobic contacts mainly dominate the interaction surface with Leu98 protruding from one monomer and extending into a hydrophobic pocket formed by Leu6, Pro7, Trp10, Phe93, Phe101, and Leu102 in the other monomer. These contacts have been shown to be important for dimer formation and thus also for protein stability.Recently, there is emerging evidence that Survivin has attracted growing attention. There are multiple strategies to utilize Survivin gene as a cancer therapeutic target as Survivin is overexpressed in cancer but undetectable in normal, differentiated Adult tissues.In this study, soughting to an approach not only efficiency but also representative, we made three truncates of Survivin at its N-terminus depending on its structure and function characterization. First, residues 6-13 in the N-terminal portion are implicated in dimerization, especially the residue Leu6, so if cut off part of them may be fail to form a dimmer. Second, residue 5-14 is an epitope for HLA-A2. According to the two points we decide to make two N-terminal deletants of SurΔN7 and SurΔN13. In addition, as all know Arg18 of Survivin is another key residue with a buried side chain that stabilizes the N-terminal part of the BIR domain, so if cut off it may be gain a truncate of Survivin no integrity of the BIR domain, which is SurΔN18. We anticipated that the three truncates SurΔN7, SurΔN13 and SurAN18 can uniformly dimerize in solution. The soluble SurF and the inclusion deletant proteins were expressed in Escherichia coli and purified using affinity chromatography.The wild-type SurF and the deletants could all fold properly and have natural secondary structure in solution. Size-exclusion chromatography results showed that in the presence of reducing agent,β-ME, SurF and all the deletants were entirely dimeric, but the elution time was slightly different as the molecular mass varied. It can be concluded that the N-terminal residues, at least up to Arg18, were not essential for homodimerization. But the single-molecule force spectroscopy (SMFS) results exhibited that the dimeric unbinding forces of the various Survivin deletants, which quantitatively (directly) measured the effect of N-terminal sequences on the strength of the dimerization. From the histogram shown, the most probable unbinding force was 82.6 pN for SurF, 62.3 pN for SurΔN13, and 60.2 pN for SurΔN18, respectively. Although the unbinding force of the deletants decreased a lot, they had enough force to form homodimer.IAP proteins play an important role in apoptosis as endogenous inhibitors of caspases. This inhibition can be relieved by a mitochondrial protein, Smac/DIABLO, which directly binds to IAPs and suppresses their function. Smac/DIABLO is the second mitochondrial protein, along with cytochrome c, released into the cytosol when cells undergo apoptosis. Unlike other IAPs, Survivin does not bind caspases directly, but it has been suggested to exert anti-apoptotic effects by physically binding to Smac/DIABLO and neutralizing its effect on other IAPs. NMR analysis has indicated that Smac/DIABLO binds across the thirdβ-strand of Survivin in solution. We also investigated the binding ability between Survivin or the deletants to Smac/DIABLO in vitro using a sandwich ELISA. Smac/DIABLO binding to SurF or the deletants increased gradually within a concentration range from 0 to 2.5μg/mL. The binding ability of SurAN7 to Smac/DIABLO was slightly lower but was generally not significantly different from SurF. The interaction of SurAN13 to Smac/DIABLO was significantly reduced but still present. However, SurAN18 completely lost the ability to bind Smac/DIABLO, and the result was the same as that of the negative control. The results from the ELISA binding assay revealed that the entire BIR domain, including the surface groove which binds Smac/DIABLO, was sensitive to alterations at the N-terminal residues 13-18, indicating that the N-terminal domain was requisite for the interaction and the dimer form was not sufficient for anti-apoptotic functions.We investigated the temperature-induced structural change; including both tertiary and secondary conformation transition of the four dimeric Survivins with different N-terminal deletion in aqueous solutions by using fluorescence, circular dichroism (CD) and Fourier transform infrared (FT-IR) spectroscopy. The target was to explore the contribution of the N-terminal residues to the whole molecule thermal stability of human Survivin.The longer N-terminal deletion, the less stability of tertiary structure was observed. In detail it should be noted that, for the transition temperature, large differences between SurAN7, SurAN13 and SurAN18, while only slight difference between SurF and SurAN7 were observed. The transition temperature of SurAN7 was 56.0℃. It was reasonable to observe the thermal stability of SurAN7 almost the same as SurF in which all the main residues participated in forming the essential hydrophobic pocket were retained only the Leu6 missing and no other important residues deletion. A slight decreasing result was observed for SurAN13, although all of the N-terminal residues (6-13) previously suggested being required for dimerization had been removed, as the interaction between two monomers decreased much which had been proved through SMFS experiments. The transition temperature of SurΔN13 was 52.0℃, which demonstrated that the N-terminal residues for dimerization were not necessary to the tertiary stability. Of note, deletion of the N-terminal sequence up to the conserved Arg18, residues determined to be important for stabilization of the BIR domain structure, resulted in a transition temperature of 46.0℃and confirmed that this protein was very instability in solution, because potentially without an intact BIR domain. In the NMR structure of the Survivin dimer, residues 14-18 were shown much influenced the BIR domain and furthermore the stability of the tertiary structure. Thus, it can be concluded that the N-terminal residues, at least up to Arg18, may be very essential for maintaining the BIR domain of human Survivin.Meanwhile, the stablity of secondary structure of SurF and the deletants varied little, that because with N-terminus deletion, the secondary structure basic invariably formed. The drastic structural transitions of proteins in aqueous solution have been revealed well by variable-variable two-dimensional (VV 2D) correlation spectra, and they have the same changed tendency. The VV 2D correlation spectra results for SurF reveals that the secondary structural variations in the intensity occured for the bands due toβ-structures (1686,1675 and 1666 cm-1), then shifts to a-helical (1653 cm-1), followed by that in the bands assigned toβ-sheet-rich aggregate (1619 cm-1) and random coil (1642 cm-1). Here, it was shown that in the main unfolding of SurF or the N-terminal deletants was initiated with the structural changes of the more temperature-sensitiveβ-sheet and P-turns, followed by the less temperature-sensitive a-helical component, and the formation ofβ-sheet-rich aggregate and disordered structure. Furthermore, the comparison of power spectra indicated that the content of 1619 cm-1 became more extensive with the longer N-terminus deletion. It could be concluded that in the main unfolding the less stability of protein render it prone to aggregate with more N-terminal residues deletion.To summarize above, we have successfully constructed three human Survivin N-terminal deletants. Our quantitative SMFS analysis showed that N-terminal deletions of Survivin caused dramatically reductions in the unbinding force of the homodimer. We have determined that the N-terminal sequences were not essential for dimer formation, but the N-terminal amino acids 13-18 were critical for Smac/DIABLO binding. We have also determined that the N-terminal sequences up to Arg18 were essential for the molecular stability. With the N-terminal deletion, the tertiary structural stability degraded, but the secondary structural stability varied a little. It could be concluded that the N-terminal residues might influence the tertiary structure but the secondary ones, although more aggregates were formed for the longer N-terminal deletants during the main unfolding.