The organization of living cells is based on networks of interacting molecules. Systematic analysis of protein interactions of 3-aa loop extension (TALE) homeodomain proteins, fundamental regulators of plant meristem function and leaf development, revealed a highly connected, complex network. The network includes nine members of Arabidopsis thaliana ovate family proteins (AtOFPs), a plant-specific protein family, indicating a close functional connection to TALE homeodomain proteins. Evidence is provided that AtOFP1 is an essential pleiotropic developmental regulator. At-OFP1 and AtOFP5 are shown to associate with the cytoskeleton and to regulate subcellular localization of TALE homeodomain proteins, suggesting a previously unrecognized control mechanism in plant development.cytoskeleton ͉ network modules ͉ protein-protein interactions ͉ BELL proteins ͉ KNOX proteins

Protein-protein interactions are fundamental to virtually every aspect of cellular functions. Blue, green and yellow bimolecular fluorescence complementation (BiFC) systems based on GFP and its variants allow the investigation of protein-protein interactions in vivo. We have developed the first red BiFC system based on an improved monomeric red fluorescent protein (mRFP1-Q66T), expanding the range of possible applications for BiFC.With interactome data available for several model organisms, a challenging next step in post-genomic research is to analyze protein complex formation in vivo. The recently developed BiFC assay is a comparably fast and simple noninvasive technology to study protein interactions inside living cells. BiFC is based on the reconstitution of the yellow fluorescent protein (YFP) from two nonfluorescent fragments when they are brought into close proximity by a physical interaction between proteins fused to each fragment 1 . This approach has proven to be robust and versatile, and multicolor versions using spectral variants of GFP further increase the potential of this technology 2 . The color spectrum available for BiFC, however, has been limited to blue, green and yellow.The red fluorescent protein from Discosoma sp. (DsRED) and its variants are established intracellular reporter proteins, but the characteristics of most DsRED variants, particularly their obligate tetramerization, impede their application in BiFC 3,4 . An extensively mutated monomeric DsRED variant (mRFP1) has been generated 5 , but substantially altered spectra, poor brightness and low photostability limit its usefulness as a reporter protein.We have identified improved mRFP1 mutants, which allowed us to establish a red fluorescent reporter system for detection of protein interactions in vivo. We used site-directed mutagenesis to generate mRFP1 variants with a randomized first position of the fluorophore. Screening of 5,000 colonies resulted in the identification of 50 clones exhibiting strong red fluorescence representing three different mutations (Q66C, Q66S and Q66T) with frequencies of 5%, 43% and 52%, respectively.

In yeast, endosomal sorting of monoubiquitylated transmembrane proteins is performed by a subset of the 19 "class E vacuolar protein sorting" proteins. The core machinery consists of 11 proteins that are organised in three complexes termed ESCRT I-III (endosomal sorting complex required for transport I-III) and is conserved in eukaryotic cells. While the pathway is well understood in yeast and animals, the plant ESCRT system is largely unexplored. At least one sequence homolog for each ESCRT component can be found in the Arabidopsis genome. Generally, sequence conservation between yeast/animals and the Arabidopsis proteins is low. To understand details about participating proteins and complex organization we have performed a systematic pairwise yeast two hybrid analysis of all Arabidopsis proteins showing homology to the ESCRT core machinery. Positive interactions were validated using bimolecular fluorescence complementation. In our experiments, most putative ESCRT components exhibited interactions with other ESCRT components that could be shown to occur on endosomes suggesting that despite their low homology to their yeast and animal counterparts they represent functional components of the plant ESCRT pathway.

The yeast two-hybrid (Y2H) method is capable of delivering vast amounts of interacting positive yeast colonies from a single library screen, particularly if a multifunctional protein is used as bait. However, the selection of definitive colonies for further molecular analysis is limited by both technical practicality and high costs. Here we demonstrate a cost-effective and simple method for the rapid selection and ranking of those Y2H-positive interaction clones that are suitable for further analysis. We performed a Y2H screen for the identification of human transforming growth factor beta2- interacting proteins in a human skin keratinocyte library. The identified clones were ranked by the amount of beta-galactosidase enzyme produced, as well as by the interaction strength of the positive colonies. The combination of high-throughput microplate fluorescence readers and specific fluorescence assays can be utilized for relative quantitation of protein-protein interaction strength of Y2H-positive colonies in crude yeast-cell lysates. We demonstrate here that the high sensitivity of the fluorescence approach can bypass cumbersome conventional methods of cell lysis used in beta-galactosidase assays, and still deliver accurate values for analysis of protein interaction data. Finally, we also achieved a better understanding of general aspects of beta-galactosidase measurements in the Y2H system, such as protein normalization, the influence of yeast culture incubation time on optimal beta-galactosidase detection, and the linearity of beta-galactosidase detection in crude cell lysates.