Protruding domain of tomato bushy stunt virus coat protein is a hitherto unrecognized class of jellyroll conformation.

The capsid protein of tomato bushy stunt virus (TBSV) has two antiparallel beta-sheet domains with the so-called jellyroll conformation. Contrary to previous analyses, we note that these domains are non-superimposable topologies. The TBSV shell (S) domain topology is common to many other proteins but the protruding (P) domain is a unique conformation so far found in no other protein. The TBSV capsid P domain did not arise from the S domain by a gene duplication event as previously assumed. It is proposed instead that the P domain was acquired from an as yet unidentified cellular protein. The four possible unique jellyroll topologies that might occur in proteins are discussed and illustrated.