Abstract
Biological moonlighting refers to proteins which express more than one function. Moonlighting proteins occur in pathogenic and commensal as well as in Gram-positive and Gram-negative bacteria. The canonical functions of moonlighting proteins are in essential cellular processes, i.e., glycolysis, protein synthesis, chaperone activity, and nucleic acid stability, and their moonlighting functions include binding to host epithelial and phagocytic cells, subepithelia, cytoskeleton as well as to mucins and circulating proteins of the immune and hemostatic systems. Sequences of the moonlighting proteins do not contain known motifs for surface export or anchoring, and it has remained open whether bacterial moonlighting proteins are actively secreted to the cell wall or whether they are released from traumatized cells and then rebind onto the bacteria. In lactobacilli, ionic interactions with lipoteichoic acids and with cell division sites are important for surface localization of the proteins. Moonlighting proteins represent an abundant class of bacterial adhesins that are part of bacterial interactions with the environment and in responses to environmental changes. Multifunctionality in bacterial surface proteins appears common: the canonical adhesion proteins fimbriae express also nonadhesive functions, whereas the mobility organelles flagella as well as surface proteases express adhesive functions.
Highlights
Concept of Moonlighting ProteinsMoonlighting proteins include an expanding class of adhesion proteins in both prokaryotic and eukaryotic organisms
It has become evident that moonlighting, or the capacity to perform biological functions not related to the canonical function assigned to the protein, is common in bacterial proteins
Moonlighting was originally considered a virulence trait in pathogenic bacteria, but the same protein species with moonlighting functions have been described in pathogens and in members of the human or the animal microbiota
Summary
Moonlighting proteins include an expanding class of adhesion proteins in both prokaryotic and eukaryotic organisms. The first identified moonlighting enzyme was glyceraldehyde 3-phosphate dehydrogenase (GAPDH)— a glycolytic enzyme—that was found in the cytoplasm as well as on the cell surface of group A streptococci This bacterium, Streptococcus pyogenes, inhabits human throat and skin and is responsible for a variety of non-invasive and invasive infections, and GAPDH was found to exhibit in vitro multiple adhesive functions with a potential to increase bacterial virulence [6]. Canonical bacterial adhesion proteins, such as the fimbriae, exhibit functions that are not directly involved in adherence to host tissues or cells, and, on the other hand, bacterial flagella as well as surface-associated proteases possess adhesive properties These proteins are not generally included in discussion of bacterial moonlighting, probably because they are multimeric and perform their functions in the same cellular compartment, i.e., the bacterial surface. Their diverse functions illustrate that multitasking is common in bacterial surface proteins, and we will end our review on a short description of fimbriae, flagella, and surface proteases
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