Bacterial Excretion of Cytoplasmic Proteins (ECP): Occurrence, Mechanism, and Function

The excretion of cytoplasmic and signal-peptide-less proteins (ECP) by microorganisms and eukaryotes remains a fascinating topic. In principle, it appears to be a waste of energy. However, it turns out that – extracellularly – some cytoplasmic proteins (CPs) exert a completely different function such as contributing to pathogenicity or evasion of the immune system. Such CPs have been referred to as ‘moonlighting' proteins. ECP is boosted by many endogenous or external factors that impair the membrane or cell wall structure. There are also differences regarding their mode of release. In some microorganisms they appear to be released directly, while in others they are embedded in membrane vesicles, or bound to the cell envelope. Some CPs might be promising candidates for vaccine developments against major bacterial pathogens.