Solution structure and function of a conserved protein SP14.3 encoded by an essential Streptococcus pneumoniae gene

Streptococcus pneumoniae is a major human pathogen that causes high mortali ty and morbidity rates and has developed resistance to many antibiotics. Th e genome of S. pneumoniae has recently been completely sequenced revealing many genes encoding hypothetical proteins of unknown function. We have foun d that the gene encoding one such conserved protein, SP14.3, is essential f or growth of S. pneumonia. Since it is essential, SP14.3 represents a poten tial target for drug discovery. Here, we describe the three-dimensional sol ution structure of SP14.3 as determined by NMR spectroscopy. The structure consists of two domains each with an alpha/beta -fold. The N-terminal domai n contains two alpha -helices and a three-stranded beta -sheet, while the C -terminal domain is composed of one alpha -helix and a five-stranded beta - sheet. The N-terminal domain of the protein contains a highly negatively ch arged surface and resembles the fold of the N-terminal domain of Thermus th ermophilus ribosomal protein S3. The C-terminal domain has a protein fold s imilar to human small nuclear ribonucleoprotein Sm D3 and Haloarcula marist mortui ribosomal protein L21E. The two domains of the protein tumble in sol ution overall as a whole with an overall molecular rotational correlation t ime (tau (m)) of 12.9 ns at 25 degreesC. The relative orientation of the tw o domains is not defined by the nuclear Overhauser effect data. Indeed, res idual dipolar couplings and the structure calculations indicate that the re lative orientation of the two domains is not rigidly oriented with respect to one another in solution. (C) 2001 Academic Press.