DNA-protein fusions and uses thereof

Details DNA-protein fusions and uses thereof DNA-protein fusions and uses thereof

1999-12-02

2002-07-09

Jones, W. Gary (Department: 1655)

Chemistry: molecular biology and microbiology

Measuring or testing process involving enzymes or...

Involving nucleic acid

C435S091100, C435S091530, C435S320100, C436S006000, C514S04400A, C536S023100, C536S023500

Reexamination Certificate

active

06416950

ABSTRACT:

BACKGROUND OF THE INVENTION
In general, the invention features DNA-protein fusions and their uses, particularly for the selection of desired proteins and their corresponding nucleic acid sequences.
Recently, a combinatorial method was developed for the isolation of proteins with desired properties from large pools of proteins (Szostak et al., U.S. Ser. No. 09/007,005; Szostak et al., WO98/31700; Roberts & Szostak, Proc. Natl. Acad. Sci. USA (1997) vol. 94, p. 12297-12302). By this method, the protein portion is linked to its encoding RNA by a covalent chemical bond. Due to the covalent nature of this linkage, selection experiments are not limited to the extremely mild reaction conditions that must be used for approaches that involve non-covalent complex formation such as ribosome display (Hanes & Plückthun, Proc. Natl. Acad. Sci. USA (1997) vol. 94, p. 4937-4942; He & Taussig, Nucl. Acids Res. (1997) vol. 25, p 5132-5143). However, precautions do need to be taken during the selection process to minimize RNA degradation, since the accidental cleavage of ribo-bonds can result in the irreversible loss of encoded information. For this reason, these selection procedures are typically carried out using reaction media and equipment that are free of ribonucleases or other deleterious contaminants.
SUMMARY OF THE INVENTION
The present invention provides methods for covalently tagging proteins with their encoding DNA sequences. These DNA-protein fusions, which may be used in molecular evolution and recognition techniques, are chemically more stable than RNA-protein fusions and therefore provide a number of advantages (as discussed in more detail below).
Accordingly, in general, the invention features methods for generating DNA-protein fusions. A first method involves: (a) linking a nucleic acid primer to an RNA molecule (preferably, at or near the RNA 3′ end) , the primer being bound to a peptide acceptor (for example, puromycin); (b) translating the RNA to produce a protein product, the protein product being covalently bound to the primer; and (c) reverse transcribing the RNA to produce a DNA-protein fusion.
A second method involves: (a) generating an RNA-protein fusion; (b) hybridizing a nucleic acid primer to the fusion (preferably, at or near the RNA 3′ end); and (c) reverse transcribing the RNA to produce a DNA-protein fusion.
In a preferred embodiment of the above methods, the method may further involve treating the product of step (c) to remove the RNA (for example, by contacting the product of step (c) with RNase H under conditions sufficient to digest the RNA). In additional preferred embodiments, the nucleic acid primer is a DNA primer; the translating step is carried out in vitro; and the nucleic acid primer has a hairpin structure. In addition, the primer may further include a photocrosslinking agent, such as psoralen, and the primer may be crosslinked to an oligonucleotide which is bound to a peptide acceptor or, alternatively, may be hybridized to the RNA molecule, followed by a linking step that is carried out by photocrosslinking.
In related aspects, the invention also features a molecule including a DNA covalently bonded to a protein (preferably, of at least 10 amino acids) through a peptide acceptor (for example, puromycin), as well as a molecule including a DNA covalently bonded to a protein, in which the protein includes at least 10 amino acids.
In preferred embodiments of both of these aspects, the protein includes at least 30 amino acids, more preferably, at least 100 amino acids, and may even include at least 200 or 250 amino acids. In other preferred embodiments, the protein is encoded by the DNA and is preferably entirely encoded by the DNA; the molecule further includes a ribonucleic acid covalently bonded to the DNA; the protein is encoded by the ribonucleic acid; and the DNA is double stranded.
In another related aspect, the invention features a population of at least 10
5
, and preferably, at least 10
14
, DNA-protein fusions of the invention, each fusion including a DNA covalently bonded to a protein.
In addition, the invention features selection methods which utilize the DNA-protein fusions described herein. A first selection method involves the steps of: (a) providing a population of DNA-protein fusions, each including a DNA covalently bonded to a candidate protein; and (b) selecting a desired DNA-protein fusion, thereby selecting the desired protein or DNA.
A second selection method involves the steps of: (a) producing a population of candidate DNA-protein fusions, each including a DNA covalently bonded to a candidate protein and having a candidate protein coding sequence which differs from a reference protein coding sequence; and (b) selecting a DNA-protein fusion having an altered function, thereby selecting the protein having the altered function or its encoding DNA.
In preferred embodiments, the selection step involves either binding of the desired protein to an immobilized binding partner or assaying for a functional activity of the desired protein. In addition, the method may further involve repeating steps (a) and (b).
In a final aspect, the invention features a solid support including an array of immobilized molecules, each including a covalently-bonded DNA-protein fusion of the invention. In a preferred embodiment, the solid support is a microchip.
As used herein, by a “population” is meant 10
5
or more molecules (for example, DNA-protein fusion molecules). Because the methods of the invention facilitate selections which begin, if desired, with large numbers of candidate molecules, a “population” according to the invention preferably means more than 10
7
molecules, more preferably, more than 10
9
, 10
13
, or 10
14
molecules, and, most preferably, more than 10
15
molecules.
By “selecting” is meant substantially partitioning a molecule from other molecules in a population. As used herein, a “selecting” step provides at least a 2-fold, preferably, a 30-fold, more preferably, a 100-fold, and, most preferably, a 1000-fold enrichment of a desired molecule relative to undesired molecules in a population following the selection step. A selection step may be repeated any number of times, and different types of selection steps may be combined in a given approach.
By a “protein” is meant any two or more naturally occurring or modified amino acids joined by one or more peptide bonds. “Protein” and “peptide” are used interchangeably herein.
By “RNA” is meant a sequence of two or more covalently bonded, naturally occurring or modified ribonucleotides. One example of a modified RNA included within this term is phosphorothioate RNA.
By “DNA” is meant a sequence of two or more covalently bonded, naturally occurring or modified deoxyribonucleotides.
By a “nucleic acid” is meant any two or more covalently bonded nucleotides or nucleotide analogs or derivatives. As used herein, this term includes, without limitation, DNA, RNA, and PNA.
By a “peptide acceptor” is meant any molecule capable of being added to the C-terminus of a growing protein chain by the catalytic activity of the ribosomal peptidyl transferase function. Typically, such molecules contain (i) a nucleotide or nucleotide-like moiety (for example, adenosine or an adenosine analog (di-methylation at the N-6 amino position is acceptable)), (ii) an amino acid or amino acid-like moiety (for example, any of the 20 D- or L-amino acids or any amino acid analog thereof (for example, O-methyl tyrosine or any of the analogs described by Ellman et al., Meth. Enzymol. 202:301, 1991), and (iii) a linkage between the two (for example, an ester, amide, or ketone linkage at the 3′ position or, less preferably, the 2′ position); preferably, this linkage does not significantly perturb the pucker of the ring from the natural ribonucleotide conformation. Peptide acceptors may also possess a nucleophile, which may be, without limitation, an amino group, a hydroxyl group, or a sulfhydryl group. In addition, peptide acceptors may be composed of nucleotide mimetics, amino acid mimetics, or mimetic

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