Evolutional And Functional Researches On The Streptomyces Coelicolor SRP Pathway

Streptomyces is one of the typical Actinobacteria, which has very complex extracellular biology, involving an extraordinary number of secreted proteins and membrane proteins. This large amount of diverse proteins has to be efficiently and accurately translocated to their destinations in a timely manner. Undoubtedly, the protein translocation systems in Streptomyces are highly evolved. Understanding its protein transportation mechanisms may lead to both scientific advances and practical applications.Unfortunately, studies in this field are still at their infancy. Inadequate knowledge about the basic characteristics of the protein translocation systems in Streptomyces hindered in-dept analyses of their transportation efficiency and regulation. Focusing on the most conserved signal recognition particle (SRP) mediated protein translocation system, this study first conducted bioinformatics analyses on the evolution and structure of the SRP system, aiming to reveal the special characteristics of Streptomyces SRP system. Then, one of these special characteristics, the unique function of the A domain of Streptomyces FtsY in membrane targeting was demonstrated by experiments. In these efforts, the main conclusions induced:1) Through the esitimation of branch- and site-specific diversification pressures, it was observed that in the early stage of Actinobacteria evolution, some residues of Ffh were under positive selection. These residues mostly located in the signal peptide-binding domain of Ffh, which indicated that the signal peptides in Actinobacteria might have undergone rapid diversification at an early history. Therefore, the highly efficient and accurate protein translocation in Streptomyces may partly due to this highly evolved signal sequence recognition machinery. 2) The protein docking technology was used to construct a theoretical model of ribosomal, nascent peptide,4.5S RNA, Ffh, FtsY, and SecYEG translocon interacting with each other. This model was highly consistent with all available biochemical evidences. It suggested a reasonable hypothesis to explain the mechanism behind the membrane-targeting function of FtsY, which indicated that Streptomyces and E. coli may be different in how the A domain of FtsY performed its membrane targeting function. 3) The critical role of an Actinobacteria-specific, strongly hydrophobic sequence located at the N-terminal of FtsY A domain in the membrane-targeting ability of FtsY was experimentally demonstrated with membrane protein extraction and Mal-PEG labeling techniques. Results suggested that this patch of amino acid residues might form a transmembrane or half-transmembrane structure that may stabilize the membrane-bound state of FtsY. This unique mechanism lended Actinobacteria FtsY better capacity to bind memberane, as compared to E. coli FtsY. Phenotypically, Actinobacteria FtsY was like an intermediate between other prokaryotic SRP receptor and eukaryotic SRP receptor. Although limited in many ways, these observations of the basic characteristics of the SRP mediated protein translocation system in Streptomyces may still offer some helpful insights to help future researches.