A group A streptococcal Enn protein potentially resulting from intergenomic recombination exhibits atypical immunoglobulin-binding characteristics

Abstract

The gene encoding the Enn protein (enn) of the M untypeable group A streptococcal (GAS) strain 64/14 was amplified by polymerase chain reaction, cloned into the expression vector pJLA602 and expressed in Escherichia coli DH5 alpha. Unlike other GAS-Enn proteins, which exhibit IgA-binding activity, the recombinant Enn enn64/14 protein reacted preferentially with human IgG3. The 1050 bp open reading frame comprising the enn64/14 gene was completely sequenced. The region of the gene encoding the signal peptide and the C-terminus exhibited > 95% homology to corresponding sections of other enn genes. The region of enn64/14 encoding the N-terminus of the mature Enn protein was found to be highly homologous to the corresponding section of the gene encoding the M-like protein of GAS serotype M9 (emmL9). The recombinant protein encoded by emmL9 was found to react with all four human IgG subclasses. About 30% of the 1152 bp open reading frame of emmL9 encoding the N-terminus was found to display > 90% homology to the corresponding section of enn64/14 but was < 50% homologous in the remainder of the gene sequence. The functional analysis of the subcloned N-terminal section of emmL9 demonstrated a polypeptide exhibiting selective binding to human IgG3. These findings suggested that enn64/14 was a hybrid gene formed by recombination of an enn gene and an emmL9 gene. The putative recombinational event could have involved a set of flanking 7 bp direct repeats. Since enn64/14 and emmL9 are genes from different phylogenetic lineages of GAS, this report provides evidence that intergenomic recombinations between different types of GAS genes can occur and could lead to hybrid proteins with unique Ig-binding characteristics.