First proposed as antimicrobial agents, histones were later recognized for their role in condensing chromosomes. Histone antimicrobial activity has been reported in innate immune responses. However, how histones kill bacteria has remained elusive. The co-localization of histones with antimicrobial peptides (AMPs) in immune cells suggests that histones may be part of a larger antimicrobial mechanism in vivo. Here we report that histone H2A enters E. coli and S. aureus through membrane pores formed by the AMPs LL-37 and magainin-2. H2A enhances AMP-induced pores, depolarizes the bacterial membrane potential, and impairs membrane recovery. Inside the cytoplasm, H2A reorganizes bacterial chromosomal DNA and inhibits global transcription. Whereas bacteria recover from the pore-forming effects of LL-37, the concomitant effects of H2A and LL-37 are irrecoverable. Their combination constitutes a positive feedback loop that exponentially amplifies their antimicrobial activities, causing antimicrobial synergy. More generally, treatment with H2A and the pore-forming antibiotic polymyxin B completely eradicates bacterial growth.

Antibiotic resistance is a global epidemic, becoming increasingly pressing due to its rapid spread. There is thus a critical need to develop new therapeutic approaches. In addition to searching for new antibiotics, looking into existing mechanisms of natural host defense may enable researchers to improve existing defense mechanisms, and to develop effective, synthetic drugs guided by natural principles. Histones, primarily known for their role in condensing mammalian DNA, are antimicrobial and share biochemical similarities with antimicrobial peptides (AMPs); however, the mechanism by which histones kill bacteria is largely unknown. Both AMPs and histones are similar in size, cationic, contain a high proportion of hydrophobic amino acids, and possess the ability to form alpha helices. AMPs, which mostly kill bacteria through permeabilization or disruption of the biological membrane, have recently garnered significant attention for playing a key role in host defenses. This chapter outlines the structure and function of histone proteins as they compare to AMPs and provides an overview of their role in innate immune responses, especially regarding the action of specific histones against 1.1 Introduction In 1922, Alexander Fleming discovered lysozyme from nasal mucus 1. This was the first human antimicrobial protein to be reported; however, the discovery of penicillin in 1928 2 overshadowed this finding, and ushered the world into the "Golden Age" of antibiotics. Recently, the rise of antibiotic resistance, combined with the stagnation in discovering new, viable antimicrobial agents, has sparked renewed interest in natural host defenses. The antimicrobial activity of histones was first reported in 1942 3 and in vitro histone killing of bacteria was further characterized in 1958 using Escherichia coli 4. However, despite originally being proposed to function as antimicrobial agents, the role of histones in condensing eukaryotic DNA became seen as their primary function and little is known about their antimicrobial role and the possible mechanisms by which they kill bacteria. The discovery that histones have a central role in innate immune responses 5 has renewed interest into understanding their antimicrobial functions. Eukaryotic organisms possess a cell nucleus and other organelles enclosed within a membrane. Their nuclei contain genetic material, typically encoded in DNA, within a nuclear envelope. Within the nucleus, small, alkaline histone proteins are used to package the DNA into 5 nm nucleosomes that condense chromatin, the chromosomal material in eukaryotic cells that is composed protein, DNA, and a small amount of RNA. The basic structural unit of chromatin is made up of 146 DNA base pairs wrapped roughly 1.5 times around a histone core. This histone core structure is made up of eight histone components: two H2A-H2B dimers and a H3-H4 tetramer 6. These core histones are highly conserved through evolution, containing the 'helix turn helix turn helix' central motif, named the histone fold, and an unstructured a...

The kinesin family proteins are often studied as prototypical molecular motors; a deeper understanding of them can illuminate regulation of intracellular transport. It is typically assumed that they function identically. Here we find that this assumption of homogeneous function appears incorrect: variation among motors' velocities in vivo and in vitro is larger than the stochastic variation expected for an ensemble of "identical" motors. When moving on microtubules, slow and fast motors are persistently slow, and fast, respectively. We develop theory that provides quantitative criteria to determine whether the observed single-molecule variation is too large to be generated from an ensemble of identical molecules. To analyze such heterogeneity, we group traces into homogeneous sub-ensembles. Motility studies varying the temperature, pH and glycerol concentration suggest at least 2 distinct functional states that are independently affected by external conditions. We end by investigating the functional ramifications of such heterogeneity through Monte-Carlo multi-motor simulations.