Genetic screen to dissect protein-protein interactions: ribonuclease inhibitor-ribonuclease A as a model system

Reversible control of enzyme-inhibitor interactions with light illumination using a photoresponsive surfactant

The effects of a photoresponsive surfactant and light illumination on the complex formed between ribonuclease A (RNase A) and a protein ribonuclease inhibitor (RI) have been investigated to develop a light-based technique to reactivate an enzyme through surfactant-induced dissociation of the enzyme-inhibitor complex. The photoresponsive surfactant undergoes a photoisomerization from the relatively hydrophobic trans isomer under visible light to the relatively hydrophilic cis isomer upon UV illumination, providing a means to reversibly control protein-inhibitor interactions. In the absence of surfactant, RI binds tightly to RNase A through noncovalent interactions, which inhibits the enzyme activity. Upon addition of the surfactant under visible light, RNase A is reactivated, regaining ~75% of the native activity in the absence of RI. In the presence of the surfactant under UV light, however, the enzyme remains inhibited. Fluorescence spectroscopy, dynamic light scattering, and circular dichroism spectroscopy reveal that RI dramatically unfolds upon addition of the trans form of the surfactant, while RNase A does not undergo noticeable structural changes under the same conditions. This indicates that RNase A reactivation occurs through dissociation of the enzyme-inhibitor complex arising from surfactant-induced unfolding of the inhibitor. As a result, photoresponsive surfactant and light illumination can be used as a novel light-based technique to dissociate enzyme-inhibitor complexes and, thus, reactivate an inhibited enzyme.

Bacteria-based in vivo peptide library screening using biopanning approach

Traditionally, library screening has been performed to identify biologically active agents including small molecules or peptides that inhibit target proteins or molecules with therapeutic interests. Due to its chemical nature, library screening is usually performed under in vitro environments using purified proteins and molecules. However, active agents identified from in vitro screenings often fail to exhibit biological activities in cells. To overcome this inherent limitation, we have developed an in vivo peptide library screening system that allows for the identification of dissociative inhibitors of protein interactions of interest. The screening is based on the reconstitution of the cI repressor from bacteriophage lambda with high-density expression peptide library and is entirely performed in bacteria cells. Furthermore, to enhance the efficacy and sensitivity of the screening, a multiple-round biopanning approach was employed for amplification and enrichment of positive peptides. Overall, this in vivo screening should provide a fast and efficient tool for identification of biologically active peptide molecules against target protein assembly.

Construction of fluorescence resonance energy transfer vectors and their application in study of structure and function of signal transducers and activators of transcription 1

Protein-protein interactions have been studied extensively by green fluorescent protein-based fluorescence resonance energy transfer (FRET). The fluorescent proteins (FP) can be fused either to the N- or C-terminus of a host protein, but it is difficult to predict which order will perturb the host protein the least and provide the largest FRET. Therefore, a researcher needs to fuse host proteins with FP at both the N- and C-termini and test every possible combination (N-N, N-C, or C-C) to promote the energy transfer efficiency. Consequently, researchers required to do many subclonings. Herein, we designed FRET vectors to make them more efficient. The expression vectors of pCFP-YFP and pYFP-CFP were constructed with both cyan fluorescent protein (CFP)-yellow fluorescent protein (YFP) and YFP-CFP coding sequences flanked by two restriction enzyme sites, and with multiple cloning regions in the middle of both coding sequences. To select an optimal combination for FRET detection, we created plasmids encoding various fusion proteins of FP and signal transducers and activators of transcription 1 (STAT1) . We found that the nuclear:cytoplasmic fluorescence intensity ratios of STAT1-FP were significantly higher than those of FP-STAT1 at steady state, and fluorescence redistribution was only observed for STAT1-FP upon interferon gamma (IFNgamma) stimulation. In addition, positive FRET signals were only detected in the C-C interactions of STAT1 homodimer. Taken together, these data indicate that fusing STAT1 at the N-terminus with FP impairs the interactions of unphosphorylated STAT1 homodimers and possibly diminishes its binding with DNA. In contrast, STAT1-FP was functional with respect to its activation. Moreover, the FRET vectors are able to facilitate FRET studies.

Antitumor effects of human ribonuclease inhibitor gene transfected on B16 melanoma cells

Human ribonuclease inhibitor (RI) is a cytoplasmic acidic protein. The experiment demonstrated that it might effectively inhibit tumor-induced angiogenesis and inhibit tumor growth. Ribonuclease inhibitor is constructed almost entirely of leucine-rich repeats, which might be involved in unknown biological effects besides inhibiting RNase A and angiogenin activities. The exact molecular mechanism of antitumor on ribonuclease inhibitor remains unclear so far. In order to further understand the function of ribonuclease inhibitor and investigate the relationship with tumor growth, our study established a transfection of human ribonuclease inhibitor cDNA into the murine B16 cells by the retroviral packaging cell line PA317. The cell line transfected with a stably high expression of ribonuclease inhibitor was identified. We found that the transfected ribonuclease inhibitor could obviously inhibit cell proliferation, regulate cell cycle and induce cell apoptosis in vitro. Mice that were injected with the B16 cells transfected RI cDNA showed a significant inhibition of the tumor growth with lighter tumor weight, lower density of microvessels, longer latent periods, and survival time than those in the other two control groups. In conclusion, the results reveal the novel mechanism that antitumor effect of ribonuclease inhibitor is also associated with inducing apoptosis, regulating cell cycle and inhibiting proliferation besides antiangiogenesis. These results suggest that ribonuclease inhibitor might be a candidate of tumor suppressor gene in some tissues. RI could become a target gene for gene therapy. Our study may be of biological and clinical importance.