Processing of cell surface lipoproteins in Streptomyces coelicolor. A new paradigm?

Bacteria can be divided into two main groups; Gram negative bacteria have two membranes surrounding the cell, the cytoplasmic membrane and the outer membrane. They also have a space between these membranes called the periplasm. Proteins which are exported across the cytoplasmic membrane typically end up in the periplasm. Gram positive bacteria have only a single membrane, surrounded by a thick cell wall. Proteins which are exported across the membrane must be anchored to the outer surface of the membrane, or the cell wall, to avoid being lost entirely from the cell. These cell surface proteins act as a point of contact with the outside world and are involved in a number of essential processes including sensing environmental changes, protein folding, respiration and nutrient scavenging. They also have an important role in surface attachment and are essential for successful host infection in disease-causing bacteria. One of the main ways in which bacteria attach proteins to their membranes is by attaching a lipid molecule to the N-terminal end of the protein. The widely accepted model suggests that these 'lipoproteins' are first targeted to the Sec (secretion) pathway by a signal sequence at the N-terminus of the protein. Sec transports them across the cytoplasmic membrane to the outside of the cell where an enzyme called Lgt attaches the lipid molecule. The signal sequence is then cleaved by an enzyme called Lsp. In Gram positive bacteria the pathway ends here and the proteins are attached to the outer face of the membrane. In Gram negative bacteria these proteins have another lipid molecule attached by an enzyme called Lnt and this allows them to be transported to the outer membrane. Only a small proportion is retained in the inner membrane. In Gram negative bacteria all the enzymes involved in lipoprotein biosynthesis are essential, probably because removing any one of them results in lipoproteins accumulating in, and destabilising, the inner membrane. In Gram positive bacteria they are essential for the virulence of many disease-causing organisms. They therefore represent an excellent target for new antibacterial drugs such as globomycin, a strong inhibitor of Lsp. We have been studying lipoprotein biosynthesis in the Gram positive bacterium Streptomyces coelicolor, and have discovered some surprising differences which could challenge the widely accepted model. Previously it was assumed that all lipoproteins were exported by Sec but this is not the case in S. coelicolor. We have good evidence that some S. coelicolor lipoproteins can be exported by the Tat secretion pathway. While Sec transports linear proteins, Tat is typically used to transport fully folded proteins which are assembled inside the cell. This challenges the current model and it also suggests that the Lgt enzyme can attach lipids to both linear and fully folded proteins. S. coelicolor is unusual in that it contains two Lgt enzymes and it is possible that one acts on Sec-, and the other on Tat-dependent lipoproteins. Finally, and perhaps most puzzlingly of all, S. coelicolor contains two Lnt enzymes. This enzyme performs a step previously thought to be unique to Gram negative bacteria. Since Gram positive bacteria do not have an outer membrane the function of these enzymes in S. coelicolor are unclear. In this study we will investigate the differences in the export and modification of Tat- and Sec-dependent lipoproteins. We will examine the functions of the two Lgt enzymes in these processing pathways. We will also test the activities of the Lnt enzymes to see if they have the same function as Lnt in Gram negative bacteria. We anticipate that the results of this study will lead to a paradigm shift in our understanding of bacterial lipoprotein biosynthesis.

Bacterial lipoproteins are specially modified proteins which are processed outside of the cell. They are exported out of the cell by a general secretion pathway known as Sec which recognises a signal sequence at the N-terminus of the protein. Once outside a lipid molecule is attached to the N-terminus by an enzyme called Lgt and the signal sequence is removed by another enzyme called Lsp. In bacteria with a single membrane (Gram positive) the process ends here and the proteins are bound to the membrane by the lipid, so that all of the protein is on the cell surface. In bacteria with two membranes (Gram negative) an enzyme called Lnt attaches another lipid molecule after the signal sequence has been removed. This second lipid molecule is involved in targeting lipoproteins to the outer membrane. In Gram negative bacteria lipoproteins are either localised to the periplasm or the cell surface. The lipoprotein processing pathway in the Gram positive bacterium Streptomyces coelicolor differs from this model. We have evidence that S. coelicolor exports some of its lipoproteins through the Tat translocase, a secretion pathway typically used to export folded proteins. It was previously assumed that the lipoprotein processing machinery could only work on unfolded proteins. S. coelicolor also contains two copies of Lgt, the enzyme which adds the first lipid molecule. It is not clear why it should need two Lgt enzymes, although they may be specific for the different (Sec and Tat) secretion pathways. S. coelicolor also has two copies of Lnt, a protein previously thought to be exclusive to Gram negative bacteria The role of the Lnt enzymes are unclear since S. coelicolor does not have an outer membrane. This project will investigate the functions of these proteins and the Tat translocase in the biosynthesis of lipoproteins in S. coelicolor. We anticipate that the findings of this study will result in a shift in our understanding of bacterial lipoprotein biosynthesis.