Intracellular/Surface Moonlighting Proteins

Bacterial surface proteins play an important role in bacterial infection and virulence. More and more intracellular proteins have been found also to be located on the cell surface where they perform a second function. The intracellular proteins that perform a second function on the cell surface were defined as intracellular/surface moonlighting proteins in this work. These proteins have been commonly identified in cell surface proteomes or cell wall proteomes in proteomics studies. Published proteomics studies were used to collect information about proteins found on the cell surface in this work. Protein sequences were obtained from UniProtKB and NCBI databases and were then analyzed by bioinformatics tools. I found that more than 1,100 out of the 3,027 proteins that were found to be located on the cell surface through proteomics studies lack transmembrane alpha-helices, transmembrane beta-barrels and signal peptides for secretion. Many of these proteins are chaperones and proteins involved in carbohydrate metabolism and biogenesis. This work shows that the presence of intracellular proteins on the cell surface is more common than previously expected. This large number of examples of intracellular/cell surface proteins will help scientists and researchers to learn more about the role the intracellular/surface moonlighting proteins play in bacterial infection and virulence. Additional information about proteins located on the cell surface can also be important in developing new vaccine or biomarkers.