Streptococcus pneumoniae open reading frame SP0082 encodes a surface protein that contains
four copies of a novel conserved repeat domain that bears no significant sequence similarity
to proteins of known function. Homologous sequences from other streptococci contain
two to six of these repeats, designated the SSURE (streptococcal surface repeat) domain.
To investigate the functional role(s) of this domain, the third SSURE repeat of SP0082 sequence
has been expressed in Escherichia coli, purified to homogeneity and characterized
by biochemical and immunological methods. The expressed protein fragment was found to
bind to fibronectin, but not to collagen or submaxillary mucin. Anti-SSURE antibodies recognized
the corresponding protein on the surface of pneumococcal cells. These data identify
S. pneumoniae SP0082 protein and its homologs in other streptococci as fibronectin-binding
surface adhesins. The SSURE domain is likely to contain a novel protein fold, which was
tentatively modeled using ab initio modeling methods.
