Moonlighting Proteins: Novel Virulence Factors In Bacterial Infections

Bacterial infection with exogenous pathogens can be thought of as a result of the evolution of specific bacterial behaviours that outwit the vast panoply of host immune defences. Such interactions occur against the background of the vast colonisation of Homo sapiens with its massive and phylogenetically-complex bacterial microbiota, whose interactions with the human host can only be guessed at. At its simplest, bacterial infection can be seen as a dynamic and evolutionarily-constrained competition between the host and the genetically-dynamic bacterial population of the environment. The major defining factor is bacterial virulence which could be defined simply as the population number required to infect a host organism. The fewer organisms required, the more virulent the organism. 

However, this has to be seen as a simplistic view of virulence, which is a systems-based phenomenon with emergent properties. Virulence is a systems-based concept which is dependent on the generation, by the bacterium, of molecules which can allow the bacterium to; (i) colonise; (ii) survive the initial colonisation process; (iii) grow and, potentially, form biofilms; (iv) defeat the approaches of the innate immune system; (v) deal with the adaptive immune cells and, finally, survive without killing the host.

The concept of virulence has given rise to the ‘virulence factor’. These will be best known in terms of the terrors of bacterial infection with gas gangrene (caused by Clostridium perfringen), the flesh-eating bacterium (mainly describing Streptococcus pyogenes), with both pathologies being caused by enzymes, and flaccid and tetanic muscle spasms caused by Clostridium botulinum and Clostridium tetani toxins, respectively. Toxins are the main factor that comes to mind when thinking of bacterial virulence. However, they are only one of a range of molecules that aid the bacterium in its colonisation and growth in the human organism. A range of other bacterial virulence factors include molecules which aid bacterial adhesion to matrices and cells, promote bacterial invasion of cells, control bacterial growth, enable bacterial evasion of host immunity, and molecules which allow bacteria to enter low growth states (e.g. dormancy) which decrease their molecular signatures in the host. The formation of the bacterial biofilm involves a range of other virulence factors including those involved in quorum sensing, biofilm dispersion and so on.

The renaissance of Bacteriology over the last 30 years (due to the upsurge in antibiotic resistance) has seen the identification of a wide range of bacterial virulence mechanisms and the discovery of a large number of molecularly-distinct virulence factors, many of which are proteins. Since the early 1990s, it has become clear that amongst these distinct virulence proteins there exist a substantial number of proteins whose main function has nothing to do with bacterial virulence. Thus cytoplasmic proteins like the glycolytic enzymes glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and enolase have been identified on the surface of a wide range of Gram positive and –negative bacteria and have been reported to have a surprising number of diverse biological actions which are assumed to contribute to bacterial virulence. Indeed, where assessed using gene inactivation/upregulaton, it has been established that these proteins have a direct role to play in bacterial virulence. These proteins are known as MOONLIGHTING PROTEINS, which are defined as proteins with more than one unique biological action. Some of the bacterial moonlighting proteins from different bacterial species, although sharing >90% sequence identity, can produce quite distinct biological actions, thus increasing the virulence ‘range’ of these proteins. 

At the time of writing, around 70 bacterial proteins have been reported to exhibit more than one biological activity, with the moonlighting function being related to some virulence phenomenon. Many of these proteins are actually found in all three domains of life, and thus can be thought of as shared signals. The discovery of the role of moonlighting proteins in bacterial interactions with their hosts is revealing the plasticity of protein evolution as it relates to protein function and to bacterial communication. Indeed there are a small number of examples of human moonlighting proteins playing a role in enhancing bacterial colonisation and virulence.

This book brings together the leading experts in the study of the pathogenicity of bacterial moonlighting proteins. The book is divided into a number of related sections. In the first, the reader is introduced to the concept of protein moonlighting and is provided with current concepts in the evolution of protein moonlighting, its structural biological underpinnings and its potential role in terms of cellular complexity and systems biology. The second section focuses on moonlighting in prokaryotes in a general sense and includes chapters on moonlighting proteins and the microbiota and the role of moonlighting bacterial proteins in autoimmunity. In the third section of this book the various moonlighting proteins that are known to function as virulence factors are discussed in some detail. This begins with the role of bacterial molecular chaperones and protein-folding catalysts in the virulence of pathogens as diverse as the Chlamydiae, Legionella pneumophila, Mycobacterium tuberculosis and Helicobacter pylori. This section, and later sections of the book, reveal both that individual bacteria can employ more than one moonlighting protein as virulence determinants and that individual proteins, such as heat shock protein (Hsp)60, and GAPDH, can function as virulence factors in more than one bacterial pathogen. Following discussions of cell stress proteins, the roles of various moonlighting proteins involved in cellular metabolism is dealt with. These include the glycolytic enzymes, GAPDH, enolase, triose phosphate isomerase and aldolase which exist on the cell surface of a range of bacteria and have a range of biological actions that are wholly unexpected. In the penultimate section the book deals with other classes of bacterial proteins including those that have the ability
to act as binding proteins, or receptors, for human cytokines. In the final section the subject switches to novel findings in bacteriophage and virus biology in which moonlighting proteins are able either to promote bacterial virulence or aid in viral infection.

This book will be of interest to a range of scientists. Clearly the major reader will be the bacteriologist/cell biologist interested in the mechanisms of bacterial virulence and in the possible role of bacterial moonlighting proteins as therapeutic targets. Given the widespread use of some of these moonlighting proteins as virulence determinants, by many of the bacterial pathogens of Homo sapiens, this is a possibility. Other readers will include immunologists, biochemists, molecular biologists and pathologists focusing on the biology of the cell stress response and those interested in the diversity of protein structure and function. The finding that bacteria encode high affinity binding proteins for key pro-inflammatory cytokines like IL-1ß and TNFa also suggests a therapeutic potential for such molecules.

Brian Henderson is Professor of Biochemistry in the Department of Microbial Diseases at the UCL-Eastman Dental Institute, University College London. He has worked in academia, both in the UK and North America, and also in the pharmaceutical and biopharmaceutical industry. He has been a cell biologist, immunologist and pharmacologist and over the past twenty years has focused on bacteria-host interactions in relation to human infection and the maintenance of the human microbiota. This is the discipline of Cellular Microbiology and Henderson published the first book on this subject in 1999. At the inception of his career as a cellular microbiologist he discovered a potent bone-destroying protein generated by a pathogenic bacterium. This protein, surprisingly, was the cell stress protein, heat shock protein (Hsp)60. This was one of the earliest bacterial moonlighting proteins discovered and is the reason that the editor has spent the last 20 years exploring the role of protein moonlighting in the life of the bacterium and its interactions with its human host. Henderson has written or edited 17 books and monographs and was the senior editor of the Cambridge University Press Monograph series: Advances in Molecular and Cellular Microbiology.