Protein A from Staphylococcus aureus plays one key role as an immobilized 
affinity ligand for the purification of antibodies. A simple method for its 
extracellular expression in Escherichia coli and subsequent purification is 
reported herein. The N-terminus of the gene coding for the five IgG binding 
domains was fused to a pelB signal peptide which is responsible for periplasmic 
localization and which is removed after translocation into the periplasmic space 
of E.coli. Different additives, which were added at the same time with the 
induction of the protein expression by IPTG, were tested in order to facilitate 
the release of the target protein. With help of this optimized release protocol, 
more than 380mgL(-1) of protein A were obtained when Tris-HCl pH 8.5 was added 
up to a final concentration of 180mM in shaking flask experiments. Based on 
these observations, a protocol was developed for the extracellular production of 
SpA in a stirred tank bioreactor yielding 5.5gL(-1) of the secreted target 
protein. After cell removal by centrifugation, the protein A-containing 
supernatant was concentrated and dialyzed by tangential flow filtration. The 
target protein was subsequently purified by anion exchange chromatography with a 
total process yield of 90% and a final purity of ⩾95% (RP HPLC) was achieved.