Chemistry: molecular biology and microbiology – Micro-organism – tissue cell culture or enzyme using process... – Recombinant dna technique included in method of making a...
Reexamination Certificate
1999-08-25
2001-10-16
Brusca, John S. (Department: 1631)
Chemistry: molecular biology and microbiology
Micro-organism, tissue cell culture or enzyme using process...
Recombinant dna technique included in method of making a...
C435S007200, C435S007210
Reexamination Certificate
active
06303337
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to non-radioactive markers that facilitate the detection and analysis of nascent proteins translated within cellular or cell-free translation systems. Nascent proteins containing these markers can be rapidly and efficiently detected, isolated and analyzed without the handling and disposal problems associated with radioactive reagents.
BACKGROUND OF THE INVENTION
Cells contain organelles, macromolecules and a wide variety of small molecules. Except for water, the vast majority of the molecules and macromolecules can be classified as lipids, carbohydrates, proteins or nucleic acids. Proteins are the most abundant cellular components and facilitate many of the key cellular processes. They include enzymes, antibodies, hormones, transport molecules and components for the cytoskeleton of the cell.
Proteins are composed of amino acids arranged into linear polymers or polypeptides. In living systems, proteins comprise over twenty common amino acids. These twenty or so amino acids are generally termed the native amino acids. At the center of every amino acid is the alpha carbon atom (C&agr;) which forms four bonds or attachments with other molecules (FIG.
1
). One bond is a covalent linkage to an amino group (NH
2
) and another to a carboxyl group (COOH) which both participate in polypeptide formation. A third bond is nearly always linked to a hydrogen atom and the fourth to a side chain which imparts variability to the amino acid structure. For example, alanine is formed when the side chain is a methyl group (—CH
3
) and a valine is formed when the side chain is an isopropyl group (—CH(CH
3
)
2
). It is also possible to chemically synthesize amino acids containing different side-chains, however, the cellular protein synthesis system, with rare exceptions, utilizes native amino acids. Other amino acids and structurally similar chemical compounds are termed non-native and are generally not found in most organisms.
A central feature of all living systems is the ability to produce proteins from amino acids. Basically, protein is formed by the linkage of multiple amino acids via peptide bonds such as the pentapeptide depicted in FIG.
1
B. Key molecules involved in this process are messenger RNA (mRNA) molecules, transfer RNA (tRNA) molecules and ribosomes (rRNA-protein complexes). Protein translation normally occurs in living cells and in some cases can also be performed outside the cell in systems referred to as cell-free translation systems. In either system, the basic process of protein synthesis is identical. The extra-cellular or cell-free translation system lo comprises an extract prepared from the intracellular contents of cells. These preparations contain those molecules which support protein translation and depending on the method of preparation, post-translational events such as glycosylation and cleavages as well. Typical cells from which cell-free extracts or in vitro extracts are made are
Escherichia coli
cells, wheat germ cells, rabbit reticulocytes, insect cells and frog oocytes.
Both in vivo and in vitro syntheses involve the reading of a sequence of bases on a mRNA molecule. The mRNA contains instructions for translation in the form of triplet codons. The genetic code specifies which amino acid is encoded by each triplet codon. For each codon which specifies an amino acid, there normally exists a cognate tRNA molecule which functions to transfer the correct amino acid onto the nascent polypeptide chain. The amino acid tyrosine (Tyr) is coded by the sequence of bases UAU and UAC, while cysteine (Cys) is coded by UGU and UGC. Variability associated with the third base of the codon is common and is called wobble.
Translation begins with the binding of the ribosome to mRNA (FIG.
2
). A number of protein factors associate with the ribosome during different phases of translation including initiation factors, elongation factors and termination factors. Formation of the initiation complex is the first step of translation. Initiation factors contribute to the initiation complex along with the mRNA and initiator tRNA (fmet and met) which recognizes the base sequence UAG. Elongation proceeds with charged tRNAs binding to ribosomes, translocation and release of the amino acid cargo into the peptide chain. Elongation factors assist with the binding of tRNAs and in elongation of the polypeptide chain with the help of enzymes like peptidyl transferase. Termination factors recognize a stop signal, such as the base sequence UGA, in the message terminating polypeptide synthesis and releasing the polypeptide chain and the mRNA from the ribosome.
The structure of tRNA is often shown as a cloverleaf representation (FIG.
3
A). Structural elements of a typical tRNA include an acceptor stem, a D-loop, an anticodon loop, a variable loop and a T&PSgr;C loop. Aminoacylation or charging of tRNA results in linking the carboxyl terminal of an amino acid to the 2′-(or 3′-) hydroxyl group of a terminal adenosine base via an ester linkage. This process can be accomplished either using enzymatic or chemical methods. Normally a particular tRNA is charged by only one specific native amino acid. This selective charging, termed here enzymatic aminoacylation, is accomplished by aminoacyl tRNA synthetases. A tRNA which selectively incorporates a tyrosine residue into the nascent polypeptide chain by recognizing the tyrosine UAC codon will be charged by tyrosine with a tyrosine-aminoacyl tRNA synthetase, while a tRNA designed to read the UGU codon will be charged by a cysteine-aminoacyl tRNA synthetase. These synthetases have evolved to be extremely accurate in charging a tRNA with the correct amino acid to maintain the fidelity of the translation process. Except in special cases where the non-native amino acid is very similar structurally to the native amino acid, it is necessary to use means other than enzymatic aminoacylation to charge a tRNA.
Molecular biologists routinely study the expression of proteins that are coded for by genes. A key step in research is to express the products of these genes either in intact cells or in cell-free extracts. Conventionally, molecular biologists use radioactively labeled amino acid residues such as
35
S-methionine as a means of detecting newly synthesized proteins or so-called nascent proteins. These nascent proteins can normally be distinguished from the many other proteins present in a cell or a cell-free extract by first separating the proteins by the standard technique of gel electrophoresis and determining if the proteins contained in the gel possess the specific radioactively labeled amino acids. This method is simple and relies on gel electrophoresis, a widely available and practiced method. It does not require prior knowledge of the expressed protein and in general does not require the protein to have any special properties. In addition, the protein can exist in a denatured or unfolded form for detection by gel electrophoresis. Furthermore, more specialized techniques such as blotting to membranes and coupled enzymatic assays are not needed. Radioactive assays also have the advantage that the structure of the nascent protein is not altered or can be restored, and thus, proteins can be isolated in a functional form for subsequent biochemical and biophysical studies.
Radioactive methods suffer from many drawbacks related to the utilization of radioactively labeled amino acids. Handling radioactive compounds in the laboratory always involves a health risk and requires special laboratory safety procedures, facilities and detailed record keeping as well as special training of laboratory personnel. Disposal of radioactive waste is also of increasing concern both because of the potential risk to the public and the lack of radioactive waste disposal sites. In addition, the use of radioactive labeling is time consuming, in some cases requiring as much as several days for detection of the radioactive label. The long time needed for such experiments is a key consideration and can seriously impede research produ
Gite Sadanand
Olejnik Jerzy
Rothschild Kenneth J.
Amber Gen. Inc.
Brusca John S.
Lundgren Jeffrey S.
Medlen & Carroll LLP
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