Purification, characterization and cloning of a novel variant of the superantigen Yersinia pseudotuberculosis-derived mitogen.

The novel superantigen designated as Yersinia pseudotuberculosis-derived mitogen typeB (YPMb) was purified from the cell lysate of Y. pseudotuberculosis O:6 strain (R-104). Proliferative response of human peripheral blood mononuclear cells to the purified YPMb was detectable at a concentration of as low as 1 pg/ml, that is comparable to the previously documented YPM (YPMa). The Vbeta repertoire specificity (3, 9, 13.1 and 13.2) of YPMb was also the same as that of YPMa. A gene (ypmB) encoding YPMb was cloned and its nucleotide sequence was determined. The open reading frame (ORF 453 bp) of the ypmB encodes a protein with 150 amino acid residues, though the precursor protein of YPMa consists of 151 amino acid residues. There is a nucleotide sequence homology of 88.9% between ypmB and ypmA. The low homology in the downstream of the structural gene between ypmB and ypmA and the difference in the GC content in the ORF of ypmB and ypmA from that of the base usage of Y. pseudotuberculosis suggest that the ypm gene originates from another organism. The alignment of the amino acid sequences of mature proteins of YPMb and YPMa revealed that there is 83% homology (108 amino acid residues are identical). Between YPMa and YPMb, the central region is less homologous than the N- and C-terminal regions. Based on the functional similarity of two superantigen molecules, the less homology suggested that the central region is less important for the function of both YPM molecules.