Making hybrids of two-hybrid systems

p53-dependent inhibition of mammalian cell survival by a genetically selected peptide aptamer that targets the regulatory subunit of protein kinase CK2

Based on the perturbation of its expression in human cancers and on its involvement in transformation and tumorigenesis, protein kinase CK2 has recently attracted attention as a potential therapeutic target. To assess the value of CK2 as a target for antiproliferative strategies, we have initiated a program aiming to develop inhibitors targeting specifically the regulatory CK2beta subunit. Here, we use a two-hybrid approach to isolate from combinatorial libraries, peptide aptamers that specifically interact with CK2beta. One of these (P1), which has significant sequence homology to the cytomegalovirus IE2 protein, binds with high affinity to the N-terminal domain of CK2beta without disrupting the formation of the CK2 holoenzyme. Expression of green fluorescent protein (GFP)-P1 in different mammalian cell lines activates p53 phosphorylation on serine 15, induces an upregulation of p21 and the release of the Cyt-C and apoptosis-inducing factor proapoptotic proteins triggering caspase-dependent and caspase-independent apoptosis. GFP-P1-induced apoptosis is associated with a p53-dependent pathway as cell death was abrogated in p53 knocked out cells. In summary, our data show that genetically selected peptide aptamers that specifically target CK2beta can induce apoptosis in mammalian cells through the recruitment of a p53-dependent apoptosis pathway. They also emphasize the critical role of CK2beta for cell survival and might allow the design of novel proapoptotic agents targeting this protein.

Interactions of phocein with nucleoside-diphosphate kinase, Eps15, and Dynamin I

Phocein, an intracellular protein interacting with striatin, bears a few homologies with the sigma-subunits of clathrin adaptor proteins (Baillat, G., Moqrich, A., Castets, F., Baude, A., Bailly, Y., Benmerah, A., and Monneron, A. (2001) Mol. Biol. Cell 12, 663-673). Using phocein as a bait in a yeast two-hybrid screen, we identified two novel interacting proteins, nucleoside-diphosphate kinase (NDPK) and Eps15. Immunoprecipitation and pull-down experiments involving native and/or recombinant phocein and, respectively, NDPK and Eps15, biochemically validated their interactions. NDPK and Eps15 were recently shown to be functional neighbors of dynamin. Dynamin I is shown here to directly interact with NDPK through its C-terminal proline-rich domain, whereas recombinant phocein associates with native dynamin I. Immunocytochemical studies of rat embryonic hippocampal neurons demonstrated partial co-localization of phocein and dynamin I. Phocein thus appears to be a component of the complexes involved in some steps of the vesicular traffic machinery.

Using the yeast interaction trap and other two-hybrid-based approaches to study protein-protein interactions

The detection of physical interaction between two or more molecules of interest can be facilitated if the act of association between the interactive partners leads to the production of a readily observed biological or physical readout. Many interacting molecule pairs (X, Y) can be made to induce such a readout if X and Y are each fused to defined protein elements with desired properties. For example, in the yeast forward two-hybrid system, X is synthesized as a translational fusion to a DNA-binding domain (DBD), Y is synthesized as a fusion to a transcriptional activation domain (AD), and coexpression of DBD-X and AD-Y induces transcription of easily scored responsive reporters. Other approaches use paradigms based on the artificial production of two, hybrid, molecules, but substitute a variety of readouts including the repression of transcription, activation of signal transduction pathways, or reconstitution of a disrupted enzymatic activity. In this article, we summarize a number of two-hybrid-based approaches, and detail the use of the forward yeast two-hybrid system in a screen to identify novel interacting partners for a protein of interest.

Interactions between Rho GTPases and Rho GDP dissociation inhibitor (Rho-GDI)

The Rho-GDP dissociation inhibitor (Rho-GDI) was used as bait in a two-hybrid screen of a human leucocyte cDNA library. Most of the isolated cDNAs encoded GTPases of the Rho subfamily: RhoA, B, C, Rac1, 2, CDC42 and RhoG. The newly discovered RhoH interacted very poorly with Rho-GDI. Another protein partner shared a homology with RhoA that points to Asp67(RhoA)-Arg68(RhoA)-Leu69(RhoA) as critical for interaction with Rho-GDI. A second screen with RhoA as bait led to the isolation of GDI only. In order to investigate the relative role of protein-protein and protein-lipid interactions between Rho GTPases and Rho-GDI, CAAX box mutants of RhoA were produced. They were found to interact with Rho-GDI as efficiently as wild type RhoA, indicating that protein-protein interactions alone lead to strong binding of the two proteins. The C-terminal polybasic region of RhoA was also shown to be a site of protein-protein interaction with Rho-GDI.

Molecular cloning and characterization of phocein, a protein found from the Golgi complex to dendritic spines

Phocein is a widely expressed, highly conserved intracellular protein of 225 amino acids, the sequence of which has limited homology to the sigma subunits from clathrin adaptor complexes and contains an additional stretch bearing a putative SH3-binding domain. This sequence is evolutionarily very conserved (80% identity between Drosophila melanogaster and human). Phocein was discovered by a yeast two-hybrid screen using striatin as a bait. Striatin, SG2NA, and zinedin, the three mammalian members of the striatin family, are multimodular, WD-repeat, and calmodulin-binding proteins. The interaction of phocein with striatin, SG2NA, and zinedin was validated in vitro by coimmunoprecipitation and pull-down experiments. Fractionation of brain and HeLa cells showed that phocein is associated with membranes, as well as present in the cytosol where it behaves as a protein complex. The molecular interaction between SG2NA and phocein was confirmed by their in vivo colocalization, as observed in HeLa cells where antibodies directed against either phocein or SG2NA immunostained the Golgi complex. A 2-min brefeldin A treatment of HeLa cells induced the redistribution of both proteins. Immunocytochemical studies of adult rat brain sections showed that phocein reactivity, present in many types of neurons, is strictly somato-dendritic and extends down to spines, just as do striatin and SG2NA.

The yeast kinesin-related protein Smy1p exerts its effects on the class V myosin Myo2p via a physical interaction

We have discovered evidence for a physical interaction between a class V myosin, Myo2p, and a kinesin-related protein, Smy1p, in budding yeast. These proteins had previously been linked by genetic and colocalization studies, but we had been unable to determine the nature of their association. We now show by two-hybrid analysis that a 69-amino acid region of the Smy1p tail interacts with the globular portion of the Myo2p tail. Deletion of this myosin-binding region of Smy1p eliminates its ability to colocalize with Myo2p and to overcome the myo2-66 mutant defects, suggesting that the interaction is necessary for these functions. Further insights about the Smy1p-Myo2p interaction have come from studies of a new mutant allele, myo2-2, which causes a loss of Myo2p localization. We report that Smy1p localization is also lost in the myo2-2 mutant, demonstrating that Smy1p localization is dependent on Myo2p. We also found that overexpression of Smy1p partially restores myo2-2p localization in a myosin-binding region-dependent manner. Thus, overexpression of Smy1p can overcome defects in both the head and tail domains of Myo2p (caused by the myo2-66 and myo2-2 alleles, respectively). We propose that Smy1p enhances some aspect of Myo2p function, perhaps delivery or docking of vesicles at the bud tip.

Yeast Two-Hybrid: State of the Art

Genome projects are approaching completion and are saturating sequence databases. This paper discusses the role of the two-hybrid system as a generator of hypotheses. Apart from this rather exhaustive, financially and labour intensive procedure, more refined functional studies can be undertaken. Indeed, by making hybrids of two-hybrid systems, customised approaches can be developed in order to attack specific function-related problems. For example, one could set-up a "differential" screen by combining a forward and a reverse approach in a three-hybrid set-up. Another very interesting project is the use of peptide libraries in two-hybrid approaches. This could enable the identification of peptides with very high specificity comparable to "real" antibodies. With the technology available, the only limitation is imagination.