Streptococcus pneumoniae type 3 encodes a protein highly similar to the human glutamate decarboxylase (GAD65).

A 2.5-kb ScaI fragment of the type 3 pneumococcal strain 406 DNA containing a 1425-nucleotide open reading frame (gadA) and encoding a 475-amino acid protein (M(r) 54,427) was characterised. The gene gadA was expressed in Salmonella typhimurium. Pulsed-field gel electrophoresis and Southern blotting analysis of DNAs prepared from several pneumococcal serotypes showed that only those clinical isolates belonging to serotype 3 harbour the gadA gene. Sequence comparison of GadA with proteins included in the data banks revealed the highest similarity with human glutamate decarboxylase (GAD65) (59% similarity, 28% identity). Auto-antibodies to GAD65 have been associated with the onset of insulin-dependent diabetes mellitus. Interestingly, several epitopes of GAD65 that have been identified as immunodominant are particularly well conserved in the pneumococcal GadA.