Directed evolution of enzymes and antibodies

Abstract

The invention relates to methods of selecting proteins, out of large libraries, having desirable characteristics. Exemplified are methods of expressing enzymes and antibodies on the surface of host cells and selecting for desired activities. These methods have the advantage of speed and ease of operation when compared with current methods. They also provide, without additional cloning, a source of significant quantities of the protein of interest.

Description

[0001] The present application is a continuation of copending U.S. patent application Ser. No. 09 / 813,444, filed Mar. 20, 2001, which is a continuation of copending U.S. patent application Ser. No. 09 / 782,672, filed Feb. 12, 2001, which is a continuation-in-part of U.S. Pat. No. 5,866,344 filed May 23, 1995 and issued Feb. 2, 1999, which is a continuation-in-part of U.S. Ser. No. 08 / 258,543, filed Jun. 10, 1994 is abandoned, which is a divisional of U.S. Pat. No. 5,348,867 issued Sep. 20, 1994. The entire text of each of the above-referenced applications and patents are specifically incorporated by reference herein without disclaimer. The government owns rights in the present invention pursuant to grant number BCS-9412502 from the National Science Foundation.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates generally to the fields of biochemistry, immunology and molecular biology. More particularly, it concerns the use of rapid selecti...

AI Technical Summary

Benefits of technology

[0015] The present invention addresses these and other drawbacks inherent in the prior art by providing new methods of screening of polypeptide libraries. For the first time it is possible to rapidly screen polypeptide libraries for potential enzymes and antibodies; often in a matter of hours. The disclosed methods allow production of large quantities of these polypeptides, potentially on a kilogram scale, from microorganism cultures. And, because selected proteins can be displayed on the surface of a host cell, assays can be conducted with remarkable rapidity.

[0019] Once generated, expression libraries are transferred, by standard methodologies, into appropriate host cells. Expression of the antibodies, enzymes or other polypeptides on the surface of host cells permits the rapid and efficient screening of libraries for the appropriate binding specificity, enzyme function or other desirable characteristic. In addition, use of appropriate label systems and substrates permits the sorting of host cells expressing proteins of interest by flow cytometery methodology (e.g., FACS).

Problems solved by technology

Presently, methods for identifying these molecules are not readily available.

However, methods of identifying antibodies and production of antibodies is often expensive, particularly where monoclonal antibodies are required.

Hybridoma technology has traditionally been employed to produce monoclonal antibodies, but these methods are time-consuming and result in isolation and production of limited numbers of specific antibodies.

Additionally, relatively small amounts of antibody are produced; consequently, hybridoma methods have not been developed for a large number of antibodies.

This is unfortunate as the potential repertoire of immunoglobulins produced in an immunized animal is quite high, on the order of >1010, yet hybridoma technology is too complicated and time consuming to adequately screen and develop large number of useful antibodies.

Unfortunately in many cases of commercial interest it is not possible to design a selection strategy.

In fact it is impossible to design selection strategies for reactions (such as Diels Alder condensation) that do not take place in biological systems.

The other limitation of mutant selection strategies is that under selective conditions cells can evolve mechanisms of survival that bypass the reaction catalyzed by the enzyme that is to be optimized.

The ability of cellular adaptation mechanisms to respond to selective pressure has frustrated efforts to direct the evolution of efficient antibody catalysts designed to complement mutations in essential pathways within microorganisms (Tang et al., 1991; Smiley and Benkovic, 1994).

However, the isolation of clones producing desired enzymes by plate-screening is tedious and is not suitable when a drastic change in protein function or stability is sought.

It is impractical to screen more than 105 clones by plate assays (even with automated techniques) and therefore only a relative small set of mutants can be analyzed.

The screening of colonies using plate assays suffers form three additional limitations.

However, neither CatELISA nor any of the other assay recent techniques can be applied to the screening of larger libraries of mutants.

Thus despite the intense interest in discovering new enzymes and antibodies, there remain a significant technical hurdles that have made it difficult to exploit this considerable wealth of biological power.

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