Streptococcal M Protein: A Common Structural Motif Used by Gram-Positive Bacteria for Biologically Active Surface Molecules

In 1981 when the coiled-coil structure of the M6 protein was determined,1 it was unique for bacterial surface molecules. Since that time, several other M proteins have been se­quenced as well as a number of other surface molecules from streptococci and a variety of other Gram-positive bacteria. At this writing, 25 surface molecules from Gram-positive organisms have been cloned and sequenced, 22 of which have now been published.2--23 A de­tailed analysis of these sequences has revealed that they possess common features which appear to be unique to Gram-positive organisms. Sequence analysis of many of these proteins has indicated that they fall into two major groups: those with repeat sequence blocks and those without.24 Conformational analysis showed that most of the molecules containing repeat sequences were found to be $-helical in those regions composed of repeats.24 Mol­ecules without repeat blocks exhibited a high degree of $-sheet, $-turn and random coil and contained limited helix potential. However, to date, there is little information to indicate that, except for M6 protein, any of these surface proteins are also in a coiled-coil conformation.