Synthetic antibodies for specific recognition and crystallization of structured RNA

Application of aptamers and autofluorescent proteins for RNA visualization

The repertoire of RNAs transcribed and processed within living cells is of extraordinary complexity. With new types of RNA being identified, the need for tools to investigate the spatio-temporal aspects of processing and trafficking of these molecules has become more evident. To visualize RNA in living cells, autofluorescent proteins (AFPs) appear as a promising alternative to synthetic fluorescent compound based labels. While current fluorescent protein-based RNA labelings have provided many new insights into the biology of RNA regulation, further improvements and adaptations are desirable to make AFP labels as valuable in the RNA world as they have proven to be for protein tagging. This article reviews the achievements and existing challenges in engineering AFPs as efficient RNA tags for high resolution fluorescence microscopy in living cells.

An engineered substance P variant for receptor-mediated delivery of synthetic antibodies into tumor cells

We have developed and tested a robust delivery method for the transport of proteins to the cytoplasm of mammalian cells without compromising the integrity of the cell membrane. This receptor-mediated delivery (RMD) technology utilizes a variant of substance P (SP), a neuropeptide that is rapidly internalized upon interaction with the neurokinin-1 receptor (NK1R). Cargos in the form of synthetic antibody fragments (sABs) were conjugated to the engineered SP variant (SPv) and efficiently internalized by NK1R-expressing cells. The sABs used here were generated to bind specific conformational forms of actin. The internalized proteins appear to escape the endosome and retain their binding activity within the cells as demonstrated by co-localization with the actin cytoskeleton. Further, since the NK1R is over-expressed in many cancers, SPv-mediated delivery provides a highly specific method for therapeutic utilization of affinity reagents targeting intracellular processes in diseased tissue.

Engineering of recombinant crystallization chaperones

The preparation of diffraction quality crystals remains the major bottleneck in macromolecular X-ray crystallography. A crystallization chaperone is an auxiliary protein, such as fragments of monoclonal antibodies, that binds to and increases the crystallization probability of a target molecule of interest. Such chaperones reduce conformational heterogeneity, mask counterproductive surfaces while extending surfaces predisposed to forming crystal contacts, and provide phasing information. Crystallization chaperones generated using recombinant technologies have emerged as superior alternatives that increase the throughput and eliminate inherent limitations associated with antibody production by animal immunization and the hybridoma technology.

The importance of being tyrosine: lessons in molecular recognition from minimalist synthetic binding proteins

Combinatorial libraries built with severely restricted chemical diversity have yielded highly functional synthetic binding proteins. Structural analyses of these minimalist binding sites have revealed the dominant role of large tyrosine residues for mediating molecular contacts and of small serine/glycine residues for providing space and flexibility. The concept of using limited residue types to construct optimized binding proteins mirrors findings in the field of small molecule drug development, where it has been proposed that most drugs are built from a limited set of side chains presented by diverse frameworks. The physicochemical properties of tyrosine make it the amino acid that is most effective for mediating molecular recognition, and protein engineers have taken advantage of these characteristics to build tyrosine-rich protein binding sites that outperform natural proteins in terms of affinity and specificity. Knowledge from preceding studies can be used to improve current designs, and thus synthetic protein libraries will continue to evolve and improve. In the near future, it seems likely that synthetic binding proteins will supersede natural antibodies for most purposes, and moreover, synthetic proteins will enable many new applications beyond the scope of natural proteins.

Crystal structure of full-length KcsA in its closed conformation

KcsA is a proton-activated, voltage-modulated K(+) channel that has served as the archetype pore domain in the Kv channel superfamily. Here, we have used synthetic antigen-binding fragments (Fabs) as crystallographic chaperones to determine the structure of full-length KcsA at 3.8 A, as well as that of its isolated C-terminal domain at 2.6 A. The structure of the full-length KcsA-Fab complex reveals a well-defined, 4-helix bundle that projects approximately 70 A toward the cytoplasm. This bundle promotes a approximately 15 degree bending in the inner bundle gate, tightening its diameter and shifting the narrowest point 2 turns of helix below. Functional analysis of the full-length KcsA-Fab complex suggests that the C-terminal bundle remains whole during gating. We suggest that this structure likely represents the physiologically relevant closed conformation of KcsA.

Overview on concepts and applications of Fab antibody fragments

In this overview, the Fab molecule is introduced and discussed as the first generated antibody fragment, which still dominates basic research and clinical applications. The unit contains a concepts section and an applications section. In the concepts section, the two principal methods for producing Fab, as well as the generation and directed evolution of Fab by phage display, are described. The applications section discusses Fab in clinical applications, as well as their increasingly important role in the determination of the three-dimensional structures of transmembrane proteins.

Chaperone-assisted crystallography with DARPins

The structure of proteins that are difficult to crystallize can often be solved by forming a noncovalent complex with a helper protein--a crystallization "chaperone." Although several such applications have been described to date, their handling usually is still very laborious. A valuable addition to the present repertoire of binding proteins is the recently developed designed ankyrin repeat protein (DARPin) technology. DARPins are built based on the natural ankyrin repeat protein fold with randomized surface residue positions allowing specific binding to virtually any target protein. The broad potential of these binding proteins for X-ray crystallography is illustrated by five cocrystal structures that have been determined recently comprising target proteins from distinct families, namely a sugar binding protein, two kinases, a caspase, and a membrane protein. This article reviews the opportunities of this technology for structural biology and the structural aspects of the DARPin-protein complexes.

Submitting antibodies to binding arbitration

For the last 30 years, the production of affinity reagents and particularly antibodies for research and therapeutic applications has been dominated by hybridoma and polyclonal technologies, while more modern, reliable and inexpensive approaches have lagged. Here we discuss why this is the case and how a cultural shift in the biomedical research community could bring the new technologies for creating antibodies and other tailor-designed binding proteins into the mainstream, with the potential for myriad new applications in research and medicine.

Unwinding RNA's secrets: advances in the biology, physics, and modeling of complex RNAs

The rapid development of our understanding of the diverse biological roles fulfilled by non-coding RNA has motivated interest in the basic macromolecular behavior, structure, and function of RNA. We focus on two areas in the behavior of complex RNAs. First, we present advances in the understanding of how RNA folding is accomplished in vivo by presenting a mechanism for the action of DEAD-box proteins. Members of this family are intimately associated with almost all cellular processes involving RNA, mediating RNA structural rearrangements and chaperoning their folding. Next, we focus on advances in understanding, and characterizing the basic biophysical forces that govern the folding of complex RNAs. Ultimately we expect that a confluence and synergy between these approaches will lead to profound understanding of RNA and its biology.