Selection of STOP-free sequences from random mutagenesis for ‘loss of interaction’ two-hybrid studies

A novel reverse two-hybrid method for the identification of missense mutations that disrupt protein-protein binding

The reverse two-hybrid system is a powerful method to select mutations that disrupt the interaction between two proteins and therefore to identify the residues involved in this interaction. However, the usefulness of this technique has been limited by its relative complexity when compared to the classical two-hybrid system, since an additional selection step is required to eliminate the high background of uninformative truncation mutants. We have developed a new method that combines the classical and reverse two-hybrid systems to select loss-of-binding missense mutations in a single step. The strategy used to select against truncation mutants is based on the two-hybrid interaction between a C-terminal fusion peptide and the Tsg101 protein. We have applied this method to identify mutations in human glucokinase (GK) that disrupt glucokinase regulatory protein (GKRP) binding. Our results indicate that this method is very efficient and eliminates all the truncation mutants and false positives. The mutated residues identified in GK are involved in the GKRP binding interface or in stabilizing the super-open conformation of GK that binds GKRP. This technique offers an improvement over existing methods in terms of speed, efficiency and simplicity and can be used to study any detectable protein interaction in the two-hybrid system.

Diversity in genetic in vivo methods for protein-protein interaction studies: from the yeast two-hybrid system to the mammalian split-luciferase system

The yeast two-hybrid system pioneered the field of in vivo protein-protein interaction methods and undisputedly gave rise to a palette of ingenious techniques that are constantly pushing further the limits of the original method. Sensitivity and selectivity have improved because of various technical tricks and experimental designs. Here we present an exhaustive overview of the genetic approaches available to study in vivo binary protein interactions, based on two-hybrid and protein fragment complementation assays. These methods have been engineered and employed successfully in microorganisms such as Saccharomyces cerevisiae and Escherichia coli, but also in higher eukaryotes. From single binary pairwise interactions to whole-genome interactome mapping, the self-reassembly concept has been employed widely. Innovative studies report the use of proteins such as ubiquitin, dihydrofolate reductase, and adenylate cyclase as reconstituted reporters. Protein fragment complementation assays have extended the possibilities in protein-protein interaction studies, with technologies that enable spatial and temporal analyses of protein complexes. In addition, one-hybrid and three-hybrid systems have broadened the types of interactions that can be studied and the findings that can be obtained. Applications of these technologies are discussed, together with the advantages and limitations of the available assays.

Random mutagenesis MAPPIT analysis identifies binding sites for Vif and Gag in both cytidine deaminase domains of Apobec3G

The mammalian two-hybrid system MAPPIT allows the detection of protein-protein interactions in intact human cells. We developed a random mutagenesis screening strategy based on MAPPIT to detect mutations that disrupt the interaction of one protein with multiple protein interactors simultaneously. The strategy was used to detect residues of the human cytidine deaminase Apobec3G that are important for its homodimerization and its interaction with the HIV-1 Gag and Vif proteins. The strategy is able to identify the previously described head-to-head homodimerization interface in the N-terminal domain of Apobec3G. Our analysis further detects two new potential interaction surfaces in the N-and C-terminal domain of Apobec3G for interaction with Vif and Gag or for Apobec3G dimerization.

A network involving Rho-type GTPases, a paxillin and a formin homologue regulates spore length and spore wall integrity in the filamentous fungus Ashbya gossypii

Fungi produce spores that allow for their dispersal and survival under harsh environmental conditions. These spores can have an astonishing variety of shapes and sizes. Using the highly polar, needle-shaped spores of the ascomycete Ashbya gossypii as a model, we demonstrated that spores produced by this organism are not simple continuous structures but rather consist of three different segments that correlate with the accumulation of different materials: a rigid tip segment, a more fragile main spore-compartment and a solid tail segment. Little is currently known about the regulatory mechanisms that control the formation of the characteristic spore morphologies. We tested a variety of mutant strains for their spore phenotypes, including spore size, shape and wall defects. The mutants that we identified as displaying such phenotypes are all known for their roles in the regulation of hyphal tip growth, including the formin protein AgBni1, the homologous Rho-type GTPases AgRho1a and AgRho1b and the scaffold protein AgPxl1. Our observations suggest that these proteins form a signalling network controlling spore length by regulating the formation of actin structures.