Regulatory role of cardiolipin in the activity of an ATP-dependent protease, Lon, from Escherichia coli

Regulation of Bacterial Two-Component Systems by Cardiolipin

The regulation of membrane protein activity for cellular functions is critically dependent on the composition of phospholipid membranes. Cardiolipin, a unique phospholipid found in bacterial membranes and mitochondrial membranes of eukaryotes, plays a crucial role in stabilizing membrane proteins and maintaining their function. In the human pathogen Staphylococcus aureus, the SaeRS two-component system (TCS) controls the expression of key virulence factors essential for the bacterium's virulence. The SaeS sensor kinase activates the SaeR response regulator via phosphoryl transfer to bind its gene target promoters. In this study, we report that cardiolipin is critical for sustaining the full activity of SaeRS and other TCSs in S. aureus. The sensor kinase protein SaeS binds directly to cardiolipin and phosphatidylglycerol, enabling SaeS activity. Elimination of cardiolipin from the membrane reduces SaeS kinase activity, indicating that bacterial cardiolipin is necessary for modulating the kinase activities of SaeS and other sensor kinases during infection. Moreover, the deletion of cardiolipin synthase genes cls1 and cls2 leads to reduced cytotoxicity to human neutrophils and lower virulence in a mouse model of infection. These findings suggest a model where cardiolipin modulates the kinase activity of SaeS and other sensor kinases after infection to adapt to the hostile environment of the host and expand our knowledge of how phospholipids contribute to membrane protein function.

DNA and Polyphosphate in Directed Proteolysis for DNA Replication Control

The strict control of bacterial cell proliferation by proteolysis is vital to coordinate cell cycle processes and to adapt to environmental changes. ATP-dependent proteases of the AAA + family are molecular machineries that contribute to cellular proteostasis. Their activity is important to control the level of various proteins, including those that are essential for the regulation of DNA replication. Since the process of proteolysis is irreversible, the protease activity must be tightly regulated and directed toward a specific substrate at the exact time and space in a cell. In our mini review, we discuss the impact of phosphate-containing molecules like DNA and inorganic polyphosphate (PolyP), accumulated during stress, on protease activities. We describe how the directed proteolysis of essential replication proteins contributes to the regulation of DNA replication under normal and stress conditions in bacteria.

Lon protease modulates virulence traits in Erwinia amylovora by direct monitoring of major regulators and indirectly through the Rcs and Gac-Csr regulatory systems

Lon, an ATP-dependent protease in bacteria, influences diverse cellular processes by degrading damaged, misfolded and short-lived regulatory proteins. In this study, we characterized the effects of lon mutation and determined the molecular mechanisms underlying Lon-mediated virulence regulation in Erwinia amylovora, an enterobacterial pathogen of apple. Erwinia amylovora depends on the type III secretion system (T3SS) and the exopolysaccharide (EPS) amylovoran to cause disease. Our results showed that mutation of the lon gene led to the overproduction of amylovoran, increased T3SS gene expression and the non-motile phenotype. Western blot analyses showed that mutation in lon directly affected the accumulation and stability of HrpS/HrpA and RcsA. Mutation in lon also indirectly influenced the expression of flhD, hrpS and csrB through the accumulation of the RcsA/RcsB proteins, which bind to the promoter of these genes. In addition, lon expression is under the control of CsrA, possibly at both the transcriptional and post-transcriptional levels. Although mutation in csrA abolished both T3SS and amylovoran production, deletion of the lon gene in the csrA mutant only rescued amylovoran production, but not T3SS. These results suggest that CsrA might positively control both T3SS and amylovoran production partly by suppressing Lon, whereas CsrA may also play a critical role in T3SS by affecting unknown targets.

Polyphosphate, cyclic AMP, guanosine tetraphosphate, and c-di-GMP reduce in vitro Lon activity

Lon protease is conserved from bacteria to humans and regulates cellular processes by degrading different classes of proteins including antitoxins, transcriptional activators, unfolded proteins, and free ribosomal proteins. Since we found that Lon has several putative cyclic diguanylate (c-di-GMP) binding sites and since Lon binds polyphosphate (polyP) and lipid polysaccharide, we hypothesized that Lon has an affinity for phosphate-based molecules that might regulate its activity. Hence we tested the effect of polyP, cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), guanosine tetraphosphate (ppGpp), c-di-GMP, and GMP on the ability of Lon to degrade α-casein. Inhibition of in vitro Lon activity occurred for polyP, cAMP, ppGpp, and c-di-GMP. We also demonstrated by HPLC that Lon is able to bind c-di-GMP. Therefore, four cell signals were found to regulate the activity of Lon protease.

Crosstalk between DnaA protein, the initiator of Escherichia coli chromosomal replication, and acidic phospholipids present in bacterial membranes

Anionic (i.e., acidic) phospholipids such as phosphotidylglycerol (PG) and cardiolipin (CL), participate in several cellular functions. Here we review intriguing in vitro and in vivo evidence that suggest emergent roles for acidic phospholipids in regulating DnaA protein-mediated initiation of Escherichia coli chromosomal replication. In vitro acidic phospholipids in a fluid bilayer promote the conversion of inactive ADP-DnaA to replicatively proficient ATP-DnaA, yet both PG and CL also can inhibit the DNA-binding activity of DnaA protein. We discuss how cellular acidic phospholipids may positively and negatively influence the initiation activity of DnaA protein to help assure chromosomal replication occurs once, but only once, per cell-cycle. Fluorescence microscopy has revealed that PG and CL exist in domains located at the cell poles and mid-cell, and several studies link membrane curvature with sub-cellular localization of various integral and peripheral membrane proteins. E. coli DnaA itself is found at the cell membrane and forms helical structures along the longitudinal axis of the cell. We propose that there is cross-talk between acidic phospholipids in the bacterial membrane and DnaA protein as a means to help control the spatial and temporal regulation of chromosomal replication in bacteria.