The Protein-Binding Potential of C2H2 Zinc Finger Domains

There are over 10,000 C2H2-type zinc finger (ZF) domains distributed among more than 1,000 ZF proteins in the human genome. These domains are frequently observed to be involved in sequence-specific DNA binding, and uncharacterized domains are typically assumed to facilitate DNA interactions. However, some ZFs also facilitate binding to proteins or RNA. Over 100 Cys2-His2 (C2H2) ZF-protein interactions have been described. We initially attempted a bioinformatics analysis to identify sequence features that would predict a DNA- or protein-binding function. These efforts were complicated by several issues, including uncertainties about the full functional capabilities of the ZFs. We therefore applied an unbiased approach to directly examine the potential for ZFs to facilitate DNA or protein interactions. The human OLF-1/EBF associated zinc finger (OAZ) protein was used as a model. The human O/E-1-associated zinc finger protein (hOAZ) contains 30 ZFs in 6 clusters, some of which have been previously indicated in DNA or protein interactions. DNA binding was assessed using a target site selection (CAST) assay, and protein binding was assessed using a yeast two-hybrid assay. We observed that clusters known to bind DNA could facilitate specific protein interactions, but clusters known to bind protein did not facilitate specific DNA interactions. Our primary conclusion is that DNA binding is a more restricted function of ZFs, and that their potential for mediating protein interactions is likely greater. These results suggest that the role of C2H2 ZF domains in protein interactions has probably been underestimated. The implication of these findings for the prediction of ZF function is discussed.     

CTCF is a uniquely versatile transcription regulator linked to epigenetics and disease

CTCF is an evolutionarily conserved zinc finger (ZF) phosphoprotein that binds through combinatorial use of its 11 ZFs to approximately 50 bp target sites that have remarkable sequence variation. Formation of different CTCF-DNA complexes, some of which are methylation-sensitive, results in distinct functions, including gene activation, repression, silencing and chromatin insulation. Disrupting the spectrum of target specificities by ZF mutations or by abnormal selective methylation of targets is associated with cancer. CTCF emerges, therefore, as a central player in networks linking expression domains with epigenetics and cell growth regulation.     

Weird fingers: Functional analysis of WIP domain proteins

WIP proteins form a plant specific subfamily of C2H2 zinc finger (ZF) proteins. In this study, we functionally characterized the WIP domain, which consists of four ZF motifs, and discuss molecular functions for WIP proteins. Mutations in each of the ZFs lead to loss of function of the TT1/WIP1 protein in Arabiopsis thaliana. SV40 type nuclear localisation signals were detected in two of the ZFs and functionally characterized using GFP fusions as well as new mutant alleles identified by TILLING. Promoter swap experiments showed that selected WIP proteins are partially able to take over TT1 function. Activity of the AtBAN promoter, a potential TT1 target, could be increased by the addition of TT1 to the TT2-TT8-TTG1 regulatory complex.     

NAR Breakthrough Article: A systematic survey of the Cys2His2 zinc finger DNA-binding landscape

Cys₂His₂ zinc fingers (C2H2-ZFs) comprise the largest class of metazoan DNA-binding domains. Despite this domain''s well-defined DNA-recognition interface, and its successful use in the design of chimeric proteins capable of targeting genomic regions of interest, much remains unknown about its DNA-binding landscape. To help bridge this gap in fundamental knowledge and to provide a resource for design-oriented applications, we screened large synthetic protein libraries to select binding C2H2-ZF domains for each possible three base pair target. The resulting data consist of >160 000 unique domain–DNA interactions and comprise the most comprehensive investigation of C2H2-ZF DNA-binding interactions to date. An integrated analysis of these independent screens yielded DNA-binding profiles for tens of thousands of domains and led to the successful design and prediction of C2H2-ZF DNA-binding specificities. Computational analyses uncovered important aspects of C2H2-ZF domain–DNA interactions, including the roles of within-finger context and domain position on base recognition. We observed the existence of numerous distinct binding strategies for each possible three base pair target and an apparent balance between affinity and specificity of binding. In sum, our comprehensive data help elucidate the complex binding landscape of C2H2-ZF domains and provide a foundation for efforts to determine, predict and engineer their DNA-binding specificities.     

Design, construction and in vitro testing of zinc finger nucleases

Zinc finger nucleases (ZFNs) are hybrid proteins that have been developed as targetable cleavage reagents for double-stranded DNA, both in vitro and in vivo. This protocol describes the design and construction of new DNA-binding domains comprised of zinc fingers (ZFs) directed at selected DNA sequences. Because the ZFNs must dimerize to cut DNA, they are designed in pairs for any new site. The first step is choosing a DNA segment of interest and searching it for sequences that can be recognized by combinations of existing ZFs. The second step is the construction of coding sequences for the selected ZF sets. Third, these coding sequences are linked to that of the nonspecific cleavage domain from the FokI restriction endonuclease in a cloning vector of choice. Finally, the ZFNs are expressed in Escherichia coli, partially purified, and tested in vitro for cleavage of the target sequences to which they were designed. If all goes smoothly, design, construction and cloning can be completed in about two weeks, with expression and testing completed in one additional week.     

De novo production of diverse intracellular antibody libraries

Many therapeutic targets are intracellular proteins and molecules designed to interact with them must effectively bind to their target inside the cell. Intracellular antibodies (intrabodies) recognise and bind to proteins in cells and various methods have been developed to produce such molecules. Intracellular antibody capture (IAC) is based on a genetic screening approach and is a facile methodology with which effective intracellular antibodies can be obtained. During the development of the IAC technology, consensus immunoglobulin variable frameworks were identified which can form the basis of intrabody libraries for direct screening. In this paper, we describe the de novo synthesis of intrabody libraries based on the IAC consensus sequence. The procedure comprises in vitro production of a single antibody gene fragment from oligonucleotides and diversification of CDRs of the immunoglobulin variable domain by mutagenic PCR. Completely de novo intrabody libraries can be rapidly generated in vitro by these approaches. As an example, a single immunoglobulin VH domain intrabody library was screened directly in yeast with an oncogenic BCR-ABL antigen bait and distinct antigen binders were isolated illustrating the functional utility of the library. This second generation IAC approach (IAC²) has many practical advantages, in particular the ability to isolate intrabodies by direct genetic selection, which obviates the need for in vitro production of antigen for pre-selection of antibody fragments.     

Characterization of the tandem CWCH2 sequence motif: a hallmark of inter-zinc finger interactions

Background  

The C2H2 zinc finger (ZF) domain is widely conserved among eukaryotic proteins. In Zic/Gli/Zap1 C2H2 ZF proteins, the two N-terminal ZFs form a single structural unit by sharing a hydrophobic core. This structural unit defines a new motif comprised of two tryptophan side chains at the center of the hydrophobic core. Because each tryptophan residue is located between the two cysteine residues of the C2H2 motif, we have named this structure the tandem CWCH2 (tCWCH2) motif.     

Role of the histidine residues of visna virus nucleocapsid protein in metal ion and DNA binding

Zinc finger (ZF) domains in retroviral nucleocapsid proteins usually contain one histidine per metal ion coordination complex (Cys-X(2)-Cys-X(4)-His-X(4)-Cys). Visna virus nucleocapsid protein, p8, has two additional histidines (in the second of its two ZFs) that could potentially bind metal ions. Absorption spectra of cobalt-bound ZF2 peptides were altered by Cys alkylation and mutation, but not by mutation of the extra histidines. Our results show that visna p8 ZFs involve three Cys and one His in the canonical spacing in metal ion coordination, and that the two additional histidines appear to interact with nucleic acid bases in p8-DNA complexes.     

The Zinc-Fingers of KREPA3 Are Essential for the Complete Editing of Mitochondrial mRNAs in Trypanosoma brucei

Most mitochondrial mRNAs in trypanosomes undergo uridine insertion/deletion editing that is catalyzed by ∼20S editosomes. The editosome component KREPA3 is essential for editosome structural integrity and its two zinc finger (ZF) motifs are essential for editing in vivo but not in vitro. KREPA3 function was further explored by examining the consequence of mutation of its N- and C- terminal ZFs (ZF1 and ZF2, respectively). Exclusively expressed myc-tagged KREPA3 with ZF2 mutation resulted in lower KREPA3 abundance and a relative increase in KREPA2 and KREL1 proteins. Detailed analysis of edited RNA products revealed the accumulation of partially edited mRNAs with less insertion editing compared to the partially edited mRNAs found in the cells with wild type KREPA3 expression. Mutation of ZF1 in TAP-tagged KREPA3 also resulted in accumulation of partially edited mRNAs that were shorter and only edited in the 3′-terminal editing region. Mutation of both ZFs essentially eliminated partially edited mRNA. The mutations did not affect gRNA abundance. These data indicate that both ZFs are essential for the progression of editing and perhaps its accuracy, which suggests that KREPA3 plays roles in the editing process via its ZFs interaction with editosome proteins and/or RNA substrates.