Bacterial small membrane proteins: the Swiss army knife of regulators at the lipid bilayer.

Small membrane proteins represent a subset of recently discovered small proteins (≤100 amino acids), which are a ubiquitous class of emerging regulators underlying bacterial adaptation to environmental stressors. Until relatively recently, small open reading frames encoding these proteins were not designated as genes in genome annotations. Therefore, our understanding of small protein biology was primarily limited to a few candidates associated with previously characterized larger partner proteins. Following the first systematic analyses of small proteins in E. coli over a decade ago, numerous small proteins have been uncovered across different bacteria. An estimated one-third of these newly discovered proteins are localized to the cell membrane, where they may interact with distinct groups of membrane proteins such as signal receptors, transporters, and enzymes, and affect their activities. Recently, there has been considerable progress in functionally characterizing small membrane protein regulators aided by innovative tools adapted specifically to study small proteins. Our review covers prototypical proteins that modulate a broad range of cellular processes such as transport, signal transduction, stress response, respiration, cell division, sporulation as well as membrane stability. Thus, small membrane proteins represent a versatile group of regulators of physiology not just at the membrane but the whole cell. Additionally, small membrane proteins have the potential for clinical applications, where some of the proteins may act as antibacterial agents themselves, while others serve as alternative drug targets for the development of novel antimicrobials.