Identification and characterization of the scl gene encoding a group a Streptococcus extracellular protein virulence factor with similarity to human collagen

Group A Streptococcus (GAS) expresses cell surface proteins that mediate im portant biological functions such as resistance to phagocytosis, adherence to plasma and extracellular matrix proteins, and degradation of host protei ns. An open reading frame encoding a protein of 348 amino acid residues was identified by analysis of the genome sequence available for a serotype M1 strain. The protein has an LPATGE sequence located near the carboxy terminu s that matches the consensus sequence (LPXTGX) present in many gram-positiv e cell wall-anchored molecules. Importantly, the central region of this pro tein contains 50 contiguous Gly-X-X triplet amino acid motifs characteristi c of the structure of human collagen. The structural gene (designated scl f or streptococcal collagen-like) was present in all 50 GAS isolates tested, which together express 21 different M protein types and represent the bread th of genomic diversity in the species. DNA sequence analysis of the gene i n these 50 isolates found that the number of contiguous Gly-X-X motifs rang ed from 14 in serotype M6 isolates to 62 in a serotype M41 organism. M1 and M18 organisms had the identical allele, which indicates very recent horizo ntal g-ne transfer. The gene was transcribed abundantly in the logarithmic but not stationary phase of growth, a result consistent with the occurrence of a DNA sequence with substantial homology with a consensus Mga binding s ite immediately upstream of the scl open reading frame. Two isogenic mutant M1 strains created by nonpolar mutagenesis of the scl structural gene were not attenuated for mouse virulence as assessed by intraperitoneal inoculat ion. In contrast, the isogenic mutant derivative made from the M1 strain re presentative of the subclone most frequently causing human infections was s ignificantly less virulent when inoculated subcutaneously into mice. In add ition, both isogenic mutant strains had significantly reduced adherence to human A549 epithelial cells grown in culture. These studies identify a new extracellular GAS virulence factor that is widely distributed in the specie s and participates in adherence to host cells and soft tissue pathology.