Expression and purification of a truncated recombinant streptococcal protein G

The gene for Protein G from Streptococcus strain G148 was cloned and expressed in Escherichia coli. The regions on the gene corresponding to the albumin-binding domains and the Fab-binding region were then deleted by site-directed mutagenesis. The translation of regions corresponding to the cell-wall- and membrane-binding domains was prevented by introduction of stop codons upstream of these domains. This recombinant DNA sequence codes for a protein (G') that contains repetitive regions and that binds only the Fc portion of IgG, analogously to Protein A. Translation of the sequence produces a protein with an Mr of about 20,000. The nucleotide sequence differs from those published previously [Guss, Eliasson, Olsson, Uhlén, Frej, Jornvall, Flock & Lindberg (1986) EMBO J. 5, 1567-1575; Olsson, Eliasson, Guss, Nilsson, Hellman, Lindberg & Uhlén (1987) Eur. J. Biochem. 168, 319-324]. The protein can be substantially purified on a large scale by chromatography on IgG-Sepharose 4B. Homogeneous Protein G' can be prepared by anion-exchange f.p.l.c. on Mono Q HR. This Protein G' has a pI of 4.19 and SDS/PAGE gives an apparent anomalous Mr of 35,000.