Streptococcal Scl1 and Scl2 proteins form collagen-like triple helices

The collagens are a family of animal proteins containing segments of repeated Gly-Xaa-Yaa (GXY) motifs that form a characteristic triple-helical structure. Genes encoding proteins with repeated GXY motifs have also been reported in bacteria and phages; however, it is unclear whether these prokaryotic proteins can form a collagen-like triple-helical structure. Here we used two recently identified streptococcal proteins, Scl1 and Scl2, containing extended GXY sequence repeats as model proteins. First we observed that prior to heat denaturation recombinant Scl proteins migrated as homotrimers in gel electrophoresis with and without SDS. We next showed that the collagen-like domain of Scl is resistant to proteolysis by trypsin. We further showed that circular dichroism spectra of the Scl proteins contained features characteristic of collagen triple helices, including a positive maximum of ellipticity at 220 nm. Furthermore the triple helices of Scl1 and Scl2 showed a temperature-dependent unfolding with melting temperatures of 36.4 and 37.6 degrees C, respectively, which resembles those seen for collagens. We finally demonstrated by electron microscopy that the Scl proteins are organized into "lollipop-like" structures, similar to those seen in human proteins with collagenous domains. This implies that the repeated GXY tripeptide motif is a structural indicator of collagen-like triple helices in proteins from such phylogenetically distant sources as bacteria and humans.