GramPos_pilinBB is one of the major backbone units of Gram-positive pili, such as those from S.pneumoniae [1]. There are three major pilin subunits that form the polymeric backbone of the pilin from S. pneumoniae, constructed of three transthyretin-like, CnaB, domains along with a crucial N-terminal domain, D1. The three Cna-B like domains are stabilised by internal Lys-Asn isopeptdie bonds, 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 [2].

1: Supramolecular organization of the repetitive backbone unit of the Streptococcus pneumoniae pilus. Spraggon G, Koesema E, Scarselli M, Malito E, Biagini M, Norais N, Emolo C, Barocchi MA, Giusti F, Hilleringmann M, Rappuoli R, Lesley S, Covacci A, Masignani V, Ferlenghi I; PLoS One. 2010;5:e10919. PMID:20559564

2: 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