GramPos_pilinD1 is the first subunit domain of Gram-positive pilins from Strep.pneumoniae. There are three major pilin subunits that form the polymeric backbone of the pilin from S. pneumoniae, constructed of three Ig-like, CnaB, domains along with a crucial N-terminal domain, D1. The three IG-like domains are stabilised by internal Lys-Asn isopeptide bonds, but this N-terminal domain makes few contact with the rest of the molecule due to the different orientation of its G beta-strand. Strand G of D1 also carries the YPKN motif that provides the essential Lys residue for the sortase-mediated intermolecular linkages along the pilus shaft. Gram-positive pili are formed from a single chain of covalently linked subunit proteins (pilins), usually comprising an adhesin at the distal tip, a major pilin that forms the polymer shaft and a minor pilin that mediates cell wall anchoring at the base [1].

1: Structure of the full-length major pilin from Streptococcus pneumoniae: implications for isopeptide bond formation in gram-positive bacterial pili. Paterson NG, Baker EN; PLoS One. 2011;6:e22095. PMID:21760959