Production and fluorescence-activated cell sorting of Escherichia coli expressing a functional antibody fragment on the external surface

We have expressed a single chain Fv (scFv) antibody fragment, consisting of the variable heavy and variable light domains from two separate anti-digoxin monoclonal antibodies, on the external surface of Escherichia coli by fusing it to an Lpp-OmpA hybrid previously shown to direct heterologous proteins to the cell surface. This scFv fusion was expressed at a high level and was shown to bind the hapten with high affinity and specificity. Whole cell ELISAs, fluorescence microscopy, protease sensitivity, and flow cytometry all confirmed that the scFv was anchored on the outer membrane and was accessible on the surface. Utilizing fluorescence-activated cell sorting, we were able to specifically enrich scFv-producing cells from a 10(5)-fold excess of control cells in only two steps. The expression of antibody fragments on the surface of E. coli is being evaluated as an attractive method for the in vitro production and selection of useful antibody fragments.

Catalytic polyclonal antibodies

The production and characterization of rabbit polyclonal antibodies that catalyze the hydrolysis of trityl protecting groups is described. The catalytic activity increased throughout the immunization regimen, exhibiting a later and steeper rise than the simple hapten-specific antibody titer. The catalytic polyclonal antibodies displayed saturation behavior, consistent with classic Michaelis-Menton kinetics. An apparent catalytic rate enhancement (kcat/kuncat) of 125 and a Km of 31 microM were measured for the polyclonal sample. Inhibition studies indicated that 12% of the antibodies either binds the hapten or is a catalyst. Analyzing polyclonal antibodies for catalytic activity is proposed as a substantially faster, more cost effective and more general way to screen new hapten designs for their relative abilities to produce antibodies with catalytic activity.

Metalloantibodies

A metalloantibody has been constructed with a coordination site for metals in the antigen binding pocket. The Zn(II) binding site from carbonic anhydrase B was used as a model. Three histidine residues have been placed in the light chain complementarity determining regions of a single chain antibody molecule. In contrast to the native protein, the mutant displayed metal-dependent fluorescence-quenching behavior. This response was interpreted as evidence for metal binding in the three-histidine site with relative affinities in the order Cu(II) greater than Zn(II) greater than Cd(II). The presence of metal cofactors in immunoglobulins should facilitate antibody catalysis of redox and hydrolytic reactions.

Sequence-specific peptide cleavage catalyzed by an antibody

Monoclonal antibodies have been induced that are capable of catalyzing specific hydrolysis of the Gly-Phe bond of peptide substrates at neutral pH with a metal complex cofactor. The antibodies were produced by immunizing with a Co(III) triethylenetetramine (trien)-peptide hapten. These antibodies as a group are capable of binding trien complexes of not only Co(III) but also of numerous other metals. Six peptides were examined as possible substrates with the antibodies and various metal complexes. Two of these peptides were cleaved by several of the antibodies. One antibody was studied in detail, and cleavage was observed for the substrates with the trien complexes of Zn(II), Ga(III), Fe(III), In(III), Cu(II), Ni(II), Lu(III), Mg(II), or Mn(II) as cofactors. A turnover number of 6 x 10(-4) per second was observed for these substrates. These results demonstrate the feasibility of the use of cofactor-assisted catalysis in an antibody binding site to accomplish difficult chemical transformations.

Nonenzymatic sequence-specific methyl transfer to single-stranded DNA

2'-Deoxyuridine 5'-triphosphate with a methylthioether moiety at the 5 position can be incorporated into a primer-template DNA complex with Klenow enzyme. Activation with CNBr at 25 degrees C, pH 5.5, followed by treatment with piperidine produces sequence-specific cleavage on the template DNA predominantly at a single guanine position. The mechanism involves methyl-group transfer from sulfur on the modified deoxyuridine of the extended primer to N-7 of guanine on the template DNA. This raises the possibility for the design and synthesis of a nonenzymatic class of sequence-specific methyltransferases for DNA.

Adenine specific DNA chemical sequencing reaction

Reaction of DNA with K2PdCl4 at pH 2.0 followed by a piperidine workup produces specific cleavage at adenine (A) residues. Product analysis revealed the K2PdCl4 reaction involves selective depurination at adenine, affording an excision reaction analogous to the other chemical DNA sequencing reactions. Adenine residues methylated at the exocyclic amine (N6) react with lower efficiency than unmethylated adenine in an identical sequence. This simple protocol specific for A may be a useful addition to current chemical sequencing reactions.