Molecular and cellular targeting in the expression of foreign polypeptides in bacteria

Improvement of downstream processing of recombinant proteins by means of genetic engineering methods

The rapid advancement of genetic engineering has allowed to produce an impressive number of proteins on a scale which would not have been achieved by classical biotechnology. At the beginning of this development research was focussed on elucidating the mechanisms of protein overexpression. The appearance of inclusion bodies may illustrate the success. In the meantime, genetic engineering is not only expected to achieve overexpression, but to improve the whole process of protein production. For downstream processing of recombinant proteins, the synthesis of fusion proteins is of primary importance. Fusion with certain proteins or peptides may protect the target protein from proteolytic degradation and may alter its solubility. Intracellular proteins may be translocated by means of fusions with signal peptides. Affinity tags as fusion complements may render protein separation and purification highly selective. These methods as well as similar ones for improving the downstream processing of proteins will be discussed on the basis of recent literature.

Solubility of proteins isolated from inclusion bodies is enhanced by fusion to maltose-binding protein or thioredoxin

When the mammalian aspartic proteinases, procathepsin D or pepsinogen, are expressed in Escherichia coli both accumulate in inclusion bodies. While pepsinogen is efficiently refolded in vitro, recovery of procathepsin D is limited by insolubility. We expressed procathepsin D and pepsinogen in E. coli, with E. coli maltose-binding protein (MBP) or thioredoxin (trx) fused to their C-termini (aspartic proteinase-MBP or aspartic proteinase-trx). The fusion proteins were still found in inclusion bodies. However, the recovery of soluble procathepsin D-MBP and procathepsin D-trx after refolding was facilitated by the bacterial fusion partners. Maltose-binding protein was more efficient than thioredoxin in increasing the recovery of soluble protein. The vector, pET23bMBPH6, can be used for general expression of heterologous proteins in E. coli. The vector includes a histidine tag at the C-terminus of MBP to allow one-step purification of the fusion proteins under denaturing conditions. After purification, the protein of interest can be cleaved from MBP with factor Xa protease and separated from the MBP partner. Refolded pepsinogen-MBP and pepsinogen-trx were enzymatically active, but procathepsin D-MBP and procathepsin D-trx were soluble but largely inactive. The results show that the limited recovery of activity upon refolding of procathepsin D is not the consequence of competing aggregation. Thus, the fusions do not necessarily facilitate native refolding, but they do enhance the recovery of soluble protein. Such fusions could provide a system to study, in soluble form, folding states which are otherwise inaccessible because of aggregation and precipitation.

Cloning and expression of two Pasteurella multocida genes in Escherichia coli

A library of cloned Pasteurella multocida (toxigenic strain 9222, serotype D2) genomic sequences was constructed in Escherichia coli by incorporating TaqI digestion fragments into the plasmid vector pUC19. Immunological screening with antibodies directed against porin H, the major protein of the P multocida outer membrane, allowed the identification of a recombinant plasmid containing a 2.9-kbp DNA insert. This plasmid encoded the synthesis of two polypeptides, p25 (25 kDa) and p28 (28 kDa) which were detected in the different compartments of the E coli transformant. The peptide p25 was more abundant in the periplasm whereas p28 was mainly found in the cell envelope and in the cytosol. Immunological analysis indicates that p25, in contrast to p28, is antigenically related to porin H of P multocida. The expression in E coli of the gene encoding p28 was enhanced by induction of the lac promoter.