SOLUBLE RECOMBINANT PROTEINS

Recombinant proteins that are not expressed in inclusion bodies either will be soluble inside the cell or, if using an excretion vector, will be extracellular (or, if E. coli is the host, possibly periplasmic). They can be purified by conventional means. In some systems, expression is so good that the desired product is the major protein present and its purification is relatively simple. In systems where the expression level is low, the purification process can be tedious, though easier, it is hoped, than isolation from the natural source. It should be remembered that a procedure developed for isolating a protein from natural sources may not work successfully with the recombinant product, because the nature of the other proteins present influences many fractionation procedures. Because of the difficulties often experienced in recombinant protein purification, a variety of vector systems have been developed in which the expressed prod-uct is a fusion protein containing an N-terminal polypeptide that simplifies purification. Such "tags" can be subsequently removed using a specific protease. A further advantage is that the expression level is dictated mainly by the transcription and translation signals for the fusion portion of the protein, which are optimized. Tags used include proteins and polypeptides for which there is a specific anti- body, binding proteins that will interact with columns containing a specific ligand, polyhistidine tags with affinity to immobilized metal columns, sequences that result in biotinylation by the host and enable purification on an avidin column, and sequences that confer insolubility under specified conditions. Unstable proteins may be modified by the molecular biological technique of site-directed mutagenesis to remove the site of instability- for instance, an oxidizable cysteine. Such techniques are appropriate for commercial production of proteins, but may of course alter natural functioning parameters. Increased thermostability can be one modification, although it is not easy to predict mutations that will improve that parameter. Thermostable proteins originating from thermophilic bacteria do not need structural modification and, if expressed in large amounts, can be purified satisfactorily in one step by simply heat-treating the extract at 70°C for 30 min, which denatures virtually all the host proteins (e.g., see Oka et al., 1989). The host bacteria used for production of recombinant proteins are usually Escherichia coli, or Bacillus subtilis; they may express proteins at 1% to over 50% of the cellular protein, depending on such variables as the source, promoter structure, and vector type. Generally the proteins are expressed intracellularly, but leader sequences for excretion may be included. In the latter case, the protein is generally excreted into the periplasmic space, which limits the amount that can be produced. Excretion from gram-positive species such as B. subtilis sends the product into the culture medium, with little feedback limitation on total expression level.