Experientia最新文献

Transcriptional repression at the yeast silent mating type loci requires the formation of a nucleoprotein complex at specific cis-acting elements called silencers, which in turn promotes the binding of a histone-associated Sir-protein complex to adjacent chromatin. A similar mechanism of long-range transcriptional repression appears to function near telomeric repeat sequences, where it has been demonstrated that Sir3p is a limiting factor for the propagation of silencing. A combined immunofluorescence/in situ hybridization method for budding yeast was developed that maintains the three-dimensional structure of the nucleus. In wild-type cells the immunostaining of Sir3p, Sir4p and Rap1 colocalizes with Y' subtelomeric sequences detected by in situ hybridization. All three antigens and the subtelomeric in situ hybridization signals are clustered in foci, which are often adjacent to, but not coincident with, nuclear pores. This colocalization of Rap1, Sir3p and Sir4p with telomeres is lost in sir mutants, and also when Sir4p is overexpressed. To test whether the natural positioning of the two HM loci, located roughly 10 and 25 kb from the ends of chromosome III, is important for silencer function, a reporter gene flanked by wild-type silencer elements was integrated at various internal sites on other yeast chromosomes. We find that integration at internal loci situated far from telomeres abrogates the ability of silencers to repress the reporter gene. Silencing can be restored by creation of a telomere at 13 kb from the reporter construct, or by insertion of 340 bp of yeast telomeric repeat sequence at this site without chromosomal truncation. Elevation of the internal nuclear pools of Sir1p, Sir3p and Sir4p can relieve the lack of repression at the LYS2 locus in an additive manner, suggesting that in wild-type cells silencer function is facilitated by its juxtaposition to a pool of highly concentrated Sir proteins, such as those created by telomere clustering.

Various adenosine triphosphate (ATP)-dependent proteases were identified within mitochondria which mediate selective mitochondrial protein degradation and fulfill crucial functions in mitochondrial biogenesis. The matrix-localized PIM1 protease, a homologue of the Escherichia coli Lon protease, is required for respiration and maintenance of mitochondrial genome integrity. Degradation of non-native polypeptides by PIM1 protease depends on the chaperone activity of the mitochondrial Hsp70 system, posing intriguing questions about the relation between the proteolytic system and the folding machinery in mitochondria. The mitochondrial inner membrane harbors two ATP-dependent metallopeptidases, the m- and the i-AAA protease, which expose their catalytic sites to opposite membrane surfaces and cooperate in the degradation of inner membrane proteins. In addition to its proteolytic activity, the m-AAA protease has chaperone-like activity during the assembly of respiratory and ATP-synthase complexes. It constitutes a quality control system in the inner membrane for membrane-embedded protein complexes.

Mechanisms mediating the inheritance of mitochondria are poorly understood, but recent studies with the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe have begun to identify components that facilitate this essential process. These components have been identified through the analysis of conditional yeast mutants that display aberrant mitochondrial distribution at restrictive conditions. The analysis of these mutants has uncovered several novel proteins that are localized either to cytoskeletal structures or to the mitochondria themselves. Many mitochondrial inheritance mutants also show altered mitochondrial morphology and defects in maintenance of the mitochondrial genome. Although some inheritance components and mechanisms appear to function specifically in certain types of cells, other conserved proteins are likely to mediate mitochondrial behavior in all eukaryotic cells.

Evidence for actin-dependent organelle movement was first obtained from studies of cytoplasmic streaming in plants. These studies, together with cell-free organelle motility studies and biophysical analyses of muscle myosin, support a model whereby organelle-associated motor molecules utilize the energy of adenosine triphosphate binding and hydrolysis to drive movement along F-actin tracks. Recent studies indicate that this mechanism for organelle movement may be responsible for organelle and vesicle movement during secretion, endocytosis and mitochondrial inheritance in a variety of eukaryotes.

A general machinery buds and fuses transport vesicles which connect intracellular compartments with each other and allow communication with the extracellular environment. Cytoplasmic coat proteins deform membranes to bud vesicles and interact directly or indirectly with cargo molecules. Compartment-specific SNAREs (SNAP receptors) on vesicles and target membranes dock vesicles and provide a scaffolding for the general fusion machinery to initiate lipid bilayer fusion.

In vitro import studies have confirmed the participation of cytosolic protein factors in the import of various precursor proteins into mitochondria. The requirement for extramitochondrial adenosine triphosphate for the import of a group of precursor proteins seems to be correlated with the chaperone activity of the cytosolic protein factors. One of the cytosolic protein factors is hsp70, which generally recognizes and binds unfolded proteins in the cytoplasm. Hsp70 keeps the newly synthesized mitochondrial precursor proteins in import-competent unfolded conformations. Another cytosolic protein factor that has been characterized is mitochondrial import stimulation factor (MSF), which seems to be specific to mitochondrial precursor proteins. MSF recognizes the mitochondrial precursor proteins, forms a complex with them and targets them to the receptors on the outer surface of mitochondria.

A group of bacterial protein toxins interfere with vesicular trafficking inside cells. Clostridial neurotoxins affect mainly the highly regulated fusion of neurotransmitter- and hormone-containing vesicles with the plasma membrane. They cleave the three SNARE proteins: VAMP, SNAP-25 and syntaxin, and this selective proteolysis results in a blockade of exocytosis. The Helicobacter pylori cytotoxin is implicated in the pathogenesis of gastroduodenal ulcers. It causes a progressive and extensive vacuolation of cells followed by necrosis, after a cytotoxin-induced alteration of membrane trafficking by late endosomes. Vacuoles originate from this compartment in a rab7-dependent process and swell because they are acidic and accumulate membrane-permeant amines.

Mitochondria possess their own ribosomes responsible for the synthesis of a small number of proteins encoded by the mitochondrial genome. In yeast, Saccharomyces cerevisiae, the two ribosomal RNAs and a single ribosomal protein, Var1, are products of mitochondrial genes, and the remaining approximately 80 ribosomal proteins are encoded in the nucleus. The mitochondrial translation system is dispensable in yeast, providing an excellent experimental model for the molecular genetic analysis of the fundamental properties of ribosomes in general as well as adaptations required for the specialized role of ribosomes in mitochondria. Recent studies of the peptidyl transferase center, one of the most highly conserved functional centers of the ribosome, and the Var1 protein, an unusual yet essential protein in the small ribosomal subunit, have provided new insight into conserved and divergent features of the mitochondrial ribosome.

c-myc was discovered as the cellular homologue of the transduced oncogene of several avian retroviruses. The gene encodes a transcription factor, which forms a heteromeric protein complex with a partner protein termed Max. In mammalian cells, Myc is a central regulator of cell proliferation and links external signals to the cell cycle machinery. Myc also induces cells to undergo apoptosis, unless specific signals provided either by cytokines or by oncogenes block the apoptotic pathway. Recent progress sheds light both on the factors regulating the function and expression of Myc and on the downstream targets in the cell cycle. Together, these findings suggest the existence of a novel signal transduction pathway regulating both apoptosis and proliferation.

The transmission of signals from the plasma membrane to the nucleus involves a number of different pathways all of which have in common protein modification. The modification is primarily in the form of phosphorylation which leads to the activation of a series of protein kinases. It is now evident that these pathways are common to stimuli that lead to mitogenic and apoptotic responses. Even the same stimuli under different physiological conditions can cause either cell proliferation or apoptosis. Activation of specific protein kinases can in some circumstances protect against cell death, while in others it protects the cell against apoptosis. Some of the pathways involved lead to activation of transcription factors and the subsequent induction of genes involved in the process of cell death or proliferation. In other cases, such as for the tumour suppressor gene product p53, activation may be initiated both at the level of gene expression or through pre-existing proteins. Yet in others, while the initial steps in the pathway are ill-defined, it is clear that downstream activation of a series of cystein proteases is instrumental in pushing the cell towards apoptosis. In this report we review the involvement of protein kinases at several different levels in the control of cell behaviour.