Table of contents
Volume 1, Issue 10, pp. 318 - 351, October 2014
The dual role of cyclin C connects stress regulated gene expression to mitochondrial dynamics
Randy Strich, Katrina F. Cooper
Following exposure to cytotoxic agents, cellular damage is first recognized by a variety of sensor mechanisms. Thenceforth, the damage signal is transduced to the nucleus to install the correct gene expression program including the induction of genes whose products either detoxify destructive compounds or repair the damage they cause. Next, the stress signal is disseminated throughout the cell to effect the appropriate changes at organelles including the mitochondria. The mitochondria represent an important signaling platform for the stress response. An initial stress response of the mitochondria is extensive fragmentation. If the damage is prodigious, the mitochondria fragment (fission) and lose their outer membrane integrity leading to the release of pro-apoptotic factors necessary for programmed cell death (PCD) execution. As this complex biological process contains many moving parts, it must be exquisitely coordinated as the ultimate decision is life or death. The conserved C-type cyclin plays an important role in executing this molecular Rubicon by coupling changes in gene expression to mitochondrial fission and PCD. Cyclin C, along with its cyclin dependent kinase partner Cdk8, associates with the RNA polymerase holoenzyme to regulate transcription. In particular, cyclin C-Cdk8 repress many stress responsive genes. To relieve this repression, cyclin C is destroyed in cells exposed to pro-oxidants and other stressors. However, prior to its destruction, cyclin C, but not Cdk8, is released from its nuclear anchor (Med13), translocates from the nucleus to the cytoplasm where it interacts with the fission machinery and is both necessary and sufficient to induce extensive mitochondria fragmentation. Furthermore, cytoplasmic cyclin C promotes PCD indicating that it mediates both mitochondrial fission and cell death pathways. This review will summarize the role cyclin C plays in regulating stress-responsive transcription. In addition, we will detail this new function mediating mitochondrial fission and PCD. Although both these roles of cyclin C are conserved, this review will concentrate on cyclin C’s dual role in the budding yeast Saccharomyces cerevisiae.
An extensive endoplasmic reticulum-localised glycoprotein family in trypanosomatids
Harriet Allison, Amanda J. O’Reilly, Jeremy Sternberg, Mark C. Field
African trypanosomes are evolutionarily highly divergent parasitic protozoa, and as a consequence the vast majority of trypanosome membrane proteins remain uncharacterised in terms of location, trafficking or function. Here we describe a novel family of type I membrane proteins which we designate ‘invariant glycoproteins’ (IGPs). IGPs are trypanosome-restricted, with extensive, lineage-specific paralogous expansions in related taxa. In T. bruceithree IGP subfamilies, IGP34, IGP40 and IGP48 are recognised; all possess a putative C-type lectin ectodomain and are ER-localised, despite lacking a classical ER-retention motif. IGPs exhibit highest expression in stumpy stage cells, suggesting roles in developmental progression, but gene silencing in mammalian infective forms suggests that each IGP subfamily is also required for normal proliferation. Detailed analysis of the IGP48 subfamily indicates a role in maintaining ER morphology, while the ER lumenal domain is necessary and sufficient for formation of both oligomeric complexes and ER retention. IGP48 is detected by antibodies from T. b. rhodesiense infected humans. We propose that the IGPs represent a trypanosomatid-specific family of ER-localised glycoproteins, with potential contributions to life cycle progression and immunity, and utilise oligomerisation as an ER retention mechanism.
On the link between cell cycle and infection of the Alphaproteobacterium Brucella abortus
Michaël Deghelt, Jean-Jacques Letesson, Xavier De Bolle
Bacteria of the Brucella genus are responsible for brucellosis, a worldwide zoonosis. These bacteria are known to have a peculiar intracellular trafficking, with a first long and non-proliferative endosomal stage and a second proliferation stage, often associated with its localization of the bacteria in the endoplasmic reticulum (ER). However, the status of the bacterial cell cycle during the non-proliferative phase was still unknown. In a recent study [Nat. Communic. 5:4366], we followed the cell cycle of B. abortus in culture and inside the host cells. In culture, B. abortus initiates the replication of its large chromosome before the small chromosome. The origin and terminator regions of these two chromosomes display distinct localization and dynamics within B. abortus. In HeLa cells and RAW264.7 macrophages, the bacteria in G1 (i.e. before the initiation of chromosomes replication) are preferentially found during the endosomal stage of the infection. During this period, growth is also arrested. The cell cycle arrest and resume during the B. abortus trafficking in host cell suggest that like the model Alphaproteobacterium Caulobacter crescentus, these bacteria are able to block their cell cycle at the G1 phase when starvation is sensed.
Divide and conquer: processive transport enables multidrug transporters to tackle challenging drugs
Nir Fluman, Eitan Bibi
Multidrug transporters are membrane proteins that catalyze efflux of antibiotics and other toxic compounds from cells, thereby conferring drug resistance on various organisms. Unlike most solute transporters that transport a single type of compound or similar analogues, multidrug transporters are extremely promiscuous. They transport a broad spectrum of dissimilar drugs and represent a serious obstacle to antimicrobial or anticancer chemotherapy. Many challenging aspects of multidrug transporters, which are unique, have been studied in detail, including their ability to interact with chemically unrelated drugs, and how they utilize energy to drive efflux of compounds that are not only structurally but electrically different. A new and surprising dimension of the promiscuous nature of multidrug transporters has been described recently: they can move long molecules through the membrane in a processive manner.