A naturally occurring split intein from the dnaE gene of Synechocystis sp. PCC6803 (Ssp DnaE intein) has been shown to mediate efficient in vivo and in vitro transsplicing in a foreign protein context. A cis-splicing Ssp DnaE intein construct displayed splicing activity similar to the trans-splicing form, which suggests that the Nand C-terminal intein fragments have a high affinity interaction. An in vitro trans-splicing system was developed that used a bacterially expressed N-terminal fragment of the Ssp DnaE intein and either a bacterially expressed or chemically synthesized intein C-terminal fragment. Unlike artificially split inteins, the Ssp DnaE intein fragments could be reconstituted in vitro under native conditions to mediate splicing as well as peptide bond cleavage. This property allowed the development of an on-column trans-splicing system that permitted the facile separation of reactants and products. Furthermore, the trans-splicing activity of the Ssp DnaE intein was successfully applied to the cyclization of proteins in vivo. Also, the isolation of the unspliced precursor on chitin resin allowed the cyclization reaction to proceed in vitro. The Ssp DnaE intein thus represents a potentially important protein for in vivo and in vitro protein manipulation.Protein splicing elements, termed inteins (1), catalyze their own excision from a primary translation product with the concomitant ligation of the flanking protein sequences (reviewed in Refs. 2-4). Inteins catalyze three highly coordinated reactions at the N-and C-terminal splice junctions (5, 6): 1) an acyl rearrangement at the N-terminal cysteine or serine; 2) a transesterification reaction between the two termini to form a branched ester or thioester intermediate; and 3) peptide bond cleavage coupled to cyclization of the intein C-terminal asparagine to free the intein. Inteins have been engineered to be versatile tools in protein purification (7-13), protein ligation (9, 10, 12, 14 -18), and in the formation of cyclic proteins and peptides (11,19,20). However, the ligation and cyclization approaches were limited by the need to generate an N-terminal cysteine and/or C-terminal thioester intermediate in vitro.In addition to inteins engineered to trans-splice (21-24), a naturally occurring split intein was recently identified in the dnaE gene encoding the catalytic subunit of DNA polymerase III of Synechocystis sp. PCC6803 (25). The N-terminal half of DnaE, followed by a 123-amino acid intein sequence, and the C-terminal half, preceded by a 36-amino acid intein sequence, are encoded by two open reading frames located more than 745 kilobases apart in the genome. When co-expressed in Escherichia coli, the two DnaE-intein fragments exhibited protein trans-splicing (25). In this report we have further investigated the cis-and trans-splicing activities of the Ssp DnaE intein in a foreign protein context. Furthermore, novel methods were developed that allow the on-column ligation of protein fragments as well as the in vivo and in vitro cyclization of p...

Interaction of a single-chain antibody fragment (scFv) with its cognate antigen while still attached to the ribosome was studied by fluorescence correlation spectroscopy (FCS). In experiments with purified scFv, FCS was capable of resolving the difference in diffusion time between free and antibody-bound labelled antigen. Ribosome-displayed antibody fragments generated by in vitro translation, in which neither the protein nor the mRNA leaves the ribosome owing to the absence of a stop codon and stabilizing buffer conditions, could be shown to specifically bind the antigen. The antibody-antigen interaction was specific, as shown by inhibition or displacement with unlabelled antigen and by control experiments with a non-cognate antibody fragment.

Beside the interaction of the antigen-presenting major histocompatibility complex with the T-cell receptor, a co-stimulatory signal is required for T-cell activation in an immune response. To reduce immune-mediated graft rejection in corneal transplantation, where topical application of drugs in ointments or eye-drops may be possible, we selected single-chain antibody fragments (scFv) with binding affinity to rat CD86 (B7.2) that inhibit the co-stimulatory signal. We produced the IgV-like domain of rat CD86 as a fusion protein in Escherichia coli by refolding from inclusion bodies. This protein was used as a target for phage display selection of scFv from HuCAL-1, a fully artificial human antibody library. Selected binding molecules were shown to specifically bind to rat CD86 and inhibit the interaction of CD86 with CD28 and CTLA4 (CD152) in flow cytometry experiments. In an assay for CD86-dependent co-stimulation, the selected scFv fragment successfully inhibited the proliferation of T-cells induced by CD86-expressing P815 cells.

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