Regulation of cell wall and capsular synthesis of Streptococcus pneumoniae and its pathogenic mechanism
Streptococcus pneumoniae is a major human respiratory pathogen that causes millions of deaths worldwide every year. The main component of the cell wall, the poly peptidoglycan, is a three-dimensional network formed by a polysaccharide chain, which is composed of β(1-4) glycosidic bond linkages of N-acetylglucosamine and N-acetylmuramic acid, cross-linking the stem peptides. During the growth of bacteria, it is necessary to maintain the homeostasis of peptidoglycan synthesis and degradation. There are various glycosyltransferases involved in the synthesis of nascent peptidoglycans, as well as a variety of glycosidase enzymes. On one hand, peptidoglycan is an Achilles heel in the process of infecting a host for the pathogens; on the other hand, it can be pruned to inhibit the host's immune response and allow the bacteria to successfully invade the host cell. Maintaining the homeostasis of peptidoglycan is important for cell survival, growth, and division, and when peptidoglycan synthase and hydrolase function and how they function need to be strictly regulated.
The capsule composed of polysaccharides outside the cell wall is not only the most important virulence factor of Streptococcus pneumoniae, but also the frontier barrier to prevent host immune phagocytosis. Proteins involved in the synthesis of capsular polysaccharides and regulating their synthesis have been important targets for the development of vaccines and antibacterial agents for the prevention and treatment of Streptococcus pneumoniae. Elucidation of the structural basis and regulatory mechanism of the capsular polysaccharide synthesis pathway will help us to understand the virulence polymorphism of Streptococcus pneumoniae and to guide the development of new vaccines and antibiotic drugs.
Since the discovery of penicillin in 1928, antibiotics have become the main tool of treating bacterial infections. Nowadays, more than 1,000,000 tons of antibiotics are produced every year in the world; and at the same time, more and more pathogenic bacteria are becoming drug resistant, some are even capable of multi-drug resistance against multiple antibiotics (and chemical agents). One of the main mechanisms by which microbes develop resistance is to inhibit drug accumulation by reducing the permeability of the drug and/or increasing the ability pump drugs out the cell. These drug efflux pumps capable of exporting drugs out of cells mainly include five family of membrane proteins. The study of the structure and function of these membrane proteins can determine the key sites of drug recognition and the molecular mechanism of drug efflux, thus achieving rational drug modification and new drug design.
Our previous results include: The structure and glycosylation mechanism of the serine repeat glycoprotein SRRP specific to the cell surface of Gram-positive bacteria, so we analyzed the crystal structure of the substrate recognition region of S. cerevisiae SRRP protein SraP, and elucidated the molecular mechanism of SraP-mediated recognition of the host for Staphylococcus aureus (PLoS Pathogens 2014). In addition, we systematically studied the glycosylation mechanism of the S. pneumoniae SRRP protein PsrP and the structure and function of related glycosyltransferases (J Biol Chem 2014). We focused on the three-dimensional structures and working mechanisms of a series of enzymes involved in the hydrolysis and remodeling of the cell wall of S. pneumoniae, including the structure and activation mechanism of the main autolysin LytA (Acta Crystal D 2015), as well as the structures and substrate specificity of late-stage cell division specific peptidoglycan hydrolase LytB (J Biol Chem 2014) and the L,D-carboxypeptidase DacB (Acta Crystal D 2015) ,which is responsible for peptidoglycan trimming. We solved the crystal structure of glycosyltransferase GlyE of Streptococcus pneumoniae SRRP protein PsrP. The glycosylation pathway of PsrP was studied by enzymology and molecular biology system, and the molecular mechanism of PsrP glycosylation was elucidated (J Biol Chem 2017 ). We also obtained the atomic resolution structure of a novel ABC transporter (Spr0693 and Spr0694-0695) in Streptococcus pneumoniae, revealing a new drug resistance mechanism for Gram-positive bacteria (Nature Comm 2018).
Transcriptional regulation concerted carbon and nitrogen metabolism, and cyanobacterial heterocyst differentiation
Cyanobacteria originated from the Archaeology of 3 billion years ago and are one of the oldest prokaryotes on the earth. It produced oxygen through photosynthesis, triggering the transformation of the earth's surface atmosphere into an oxidizing environment 2.6 billion years ago. Its long history and tenacious vitality are mainly due to its precise regulation of the metabolic balance of carbon and nitrogen, the two essential elements needed to sustain life. We focus on the cyanobacteria Synechocystis sp. PCC 6803 and the cyanobacteria Anabaena sp. PCC 7120, respectively, to study the structures and functions of a number of key transcription factors (such as NtcA, NtcB, HetR, CcmR, CmpR, etc.) in order to decipher the molecular mechanism of transcriptional regulation concerted carbon and nitrogen metabolism in cyanobacteria. In addition to its important basic biological significance, our research has broad application prospects. It not only provides a theoretical basis for the study of algal bloom pollution caused by cyanobacteria, but also guides the improvement of plant resistance and the increase of yield based on balancing the carbon and nitrogen metabolisms.
Our previous results include: the crystal structure and allosteric mechanism of cyanobacterial nitrogen metabolism regulator NtcA (PNAS 2010); three-dimensional structure of PII-PipX complex (J Mol Biol 2010; PNAS 2013); DNA binding pattern and the mechanism of action loss for the heterocyst’s developmental regulator HetR (Scientific Reports 2015); and the three-dimensional structures and catalytic mechanisms of a series of glycosyltransferases and isomerases in the polysaccharide synthesis pathway (Glycobiology 2016)