Ever since the invention of monoclonal antibodies in 1975 and, more recently, the development of various in vitro antibody display technologies, antibodies have become one of the most powerful tools in biological research and are presently the fastest growing category of new drug entities. One molecular format that shows great promise is the intracellular antibody or intrabody that exploits the specificity and diversity of immunoglobulins to target a wide range of intracellular proteins by expressing the antibody in vivo. In principle, whatever can be achieved by a monoclonal antibody in the extracellular environment can be similarly achieved inside of a cell using an intrabody. Since intrabody synthesis can be constitutive or inducible, the level of inactivation can be toggled which might allow for a wider range of phenotypes than can be observed with gene deletion, antisense or RNAi-based knockdown strategies. Further, since intrabodies are proteins, they possess a much longer half-life compared to RNA and are also more specific to their target molecules. Also, it is possible to design or engineer intrabodies to block certain domains of a particular target protein, thus allowing for the decoupling of multiple protein activities of a single target. This might prove particularly useful for essential targets that have more than one cellular activity. Finally, since intrabodies can be multivalent, simultaneous functional knockout of two or more cellular targets is possible. Based on the above features, intrabodies are expected to play an important and immediate role for target identification and validation in functional genomics and/or proteomics. The long-term objective of this research effort is to develop a proteome-wide repertoire of intrabodies for probing and modulating protein activities inside living cells. The objective of this particular application, which is the first step towards our long-term goal, is to create a novel platform technology based on the bacterial twin-arginine translocation (Tat) pathway that enables rapid, one-step genetic selection of single-chain intrabodies against virtually any intracellular target protein. To accomplish the overall objective of this application, the following specific aims are proposed: (1) develop a genetic selection based on unique mechanistic features of the bacterial Tat system for isolating intrabody-antigen pairings;and (2) engineer intrabodies that specifically inhibit biological processes. Intrabodies are an emerging class of antibody molecules that function (e.g., bind their cognate antigen) intracellularly and, owing to their specificity and diversity, have the potential to block, suppress, alter or even enhance a vast array of biological processes. Therefore, the focus of these studies is to develop a technology platform for rapid, large-scale synthesis of intrabodies that could be used as (i) functional genomics reagents that enable characterization of novel gene products and validation of these gene products as potential drug targets and (ii) drug entities that be used in the treatment of human disorders such as cancer, AIDS or neuro-degenerative disorders.