PP2A as a master regulator of the cell cycle

Protein phosphatase 2A (PP2A) plays a critical multi-faceted role in the regulation of the cell cycle. It is known to dephosphorylate over 300 substrates involved in the cell cycle, regulating almost all major pathways and cell cycle checkpoints. PP2A is involved in such diverse processes by the formation of structurally distinct families of holoenzymes, which are regulated spatially and temporally by specific regulators. Here, we review the involvement of PP2A in the regulation of three cell signaling pathways: wnt, mTOR and MAP kinase, as well as the G1→S transition, DNA synthesis and mitotic initiation. These processes are all crucial for proper cell survival and proliferation and are often deregulated in cancer and other diseases.     

Coupling of protein localization and cell movements by a dynamically localized response regulator in Myxococcus xanthus

Myxococcus xanthus cells harbor two motility machineries, type IV pili (Tfp) and the A-engine. During reversals, the two machineries switch polarity synchronously. We present a mechanism that synchronizes this polarity switching. We identify the required for motility response regulator (RomR) as essential for A-motility. RomR localizes in a bipolar, asymmetric pattern with a large cluster at the lagging cell pole. The large RomR cluster relocates to the new lagging pole in parallel with cell reversals. Dynamic RomR localization is essential for cell reversals, suggesting that RomR relocalization induces the polarity switching of the A-engine. The analysis of RomR mutants shows that the output domain targets RomR to the poles and the receiver domain is essential for dynamic localization. The small GTPase MglA establishes correct RomR polarity, and the Frz two-component system regulates dynamic RomR localization. FrzS localizes with Tfp at the leading pole and relocates in an Frz-dependent manner to the opposite pole during reversals; FrzS and RomR localize and oscillate independently. The Frz system synchronizes these oscillations and thus the synchronous polarity switching of the motility machineries.     

A dynamically changing intracellular water network serves as a universal regulator of the cell: the water-governed cycle

The functioning of enzymes and protein folding are well known to be assisted by the surrounding chaperoning water molecules, which are connected to the protein via non-covalent, dynamically changing chemical bonds. A molecular intracellular network of weak non-covalent connections may be presumed to exist in living cells. The roles of such non-covalent networks are examined in terms of a molecular model which postulates a universal enzyme and biochemical mechanism regulating the maintenance of chemical stability in living cells.     

A dynamically localized histidine kinase controls the asymmetric distribution of polar pili proteins

Each cell division in Caulobacter crescentus is asymmetric, yielding a swarmer cell with several polar pili and a non-piliated stalked cell. To identify factors contributing to the asymmetric biogenesis of polar pili, cytological studies of pilus assembly components were performed. We show here that the CpaC protein, which is thought to form the outer membrane pilus secretion channel, and its assembly factor, CpaE, are localized to the cell pole prior to the polymerization of the pilus filament. We demonstrate that the PleC histidine kinase, a two-component signal transduction protein shown previously to localize to the piliated cell pole before and during pilus assembly, controls the accumulation of the pilin subunit, PilA. Using an inactive form of PleC (PleCH610A) that lacks the catalytic histidine residue, we provide evidence that PleC activity is responsible for the asymmetric distribution of CpaE and itself to only one of the two cell poles. Thus, a polar signal transduction protein controls its own asymmetric location as well as that of a factor assembling a polar organelle.     

Cell cycle control: a complex issue

Do p27Kip1 and p21Cip1 function as activators or inhibitors of D cyclin-cdk4 activity? Attempts to answer this question, and thus to understand how cdk4--a key cell cycle regulator--becomes active, have produced conflicting data. In this perspective, we summarize the results of studies addressing the effects of p27Kip1 and p21Cip1 on the assembly and activation of D cyclin-cdk4 complexes. Emphasis is placed on our experimental findings that support a model of cell cycle control in which p27Kip1 and p21Cip1 stabilize D cyclin-cdk4 complexes but inhibit D cyclin-cdk4 activity.     

A new regulator of the cell cycle

The ability of eukaryotes to alter chromatin structure and function is modulated, in part, by histone-modifying enzymes and the post-translational modifications they create. One of these enzymes, PR-Set7/Set8/KMT5a, is the sole histone methyltransferase responsible for the monomethylation of histone H4 lysine 20 (H4K20me1) in higher eukaryotes. Both PR-Set7 and H4K20me1 were previously found to be tightly cell cycle regulated suggesting that they play an important, although unknown, role in cell cycle progression. Several recent reports reveal that PR-Set7 abundance is dynamically regulated during different cell cycle phases by distinct enzymes including cdk1/cyclinB, Cdc14, SCF^Skp2, CRL4^cdt2 and APC^cdh1. Importantly, these reports demonstrate that inappropriate levels of PR-Set7 result in profound cell cycle defects including the inability to initiate S phase, the re-replication of DNA and the improper timing of mitotic progression. Here, we summarize the significance of these new findings, raise some important questions that require further investigation and explore several possibilities of how PR-Set7 and methylated H4K20 may likely function as novel regulators of the cell cycle.     

Heparanase stimulation of protease expression implicates it as a master regulator of the aggressive tumor phenotype in myeloma

High levels of heparanase are an indicator of poor prognosis in myeloma patients, and up-regulation of the enzyme enhances tumor growth, angiogenesis, and metastasis in animal models. At least part of the impact of heparanase in driving the aggressive tumor phenotype is due to its effect on increasing the expression and shedding of the heparan sulfate proteoglycan syndecan-1, a molecule known to promote myeloma progression. The present work demonstrated that elevation in heparanase expression in myeloma cells stimulates sustained ERK phosphorylation that in turn drives MMP-9 expression. In addition, urokinase-type plasminogen activator (uPA) and uPA receptor expression levels increased, and blocking the proteolytic activation of either MMP-9 or uPA inhibited the heparanase-induced increase in syndecan-1 shedding. Together these data provide a mechanism for heparanase-induced syndecan-1 shedding and, more importantly, demonstrate that heparanase activity in myeloma cells can lead to increased levels of proteases that are known to play important roles in the aggressive behavior of myeloma tumors. This in addition to its other known biological roles, indicates that heparanase acts as a master regulator of the aggressive tumor phenotype by up-regulating protease expression and activity within the tumor microenvironment.     

Acetyl-coenzyme A: A metabolic master regulator of autophagy and longevity

As the major lysosomal degradation pathway, autophagy represents the guardian of cellular homeostasis, removing damaged and potentially harmful material and replenishing energy reserves in conditions of starvation. Given its vast physiological importance, autophagy is crucially involved in the process of aging and associated pathologies. Although the regulation of autophagy strongly depends on nutrient availability, specific metabolites that modulate autophagic responses are poorly described. Recently, we revealed nucleo-cytosolic acetyl-coenzyme A (AcCoA) as a phylogenetically conserved inhibitor of starvation-induced and age-associated autophagy. AcCoA is the sole acetyl-group donor for protein acetylation, explaining why pharmacological or genetic manipulations that modify the concentrations of nucleo-cytosolic AcCoA directly affect the levels of protein acetylation. The acetylation of histones and cytosolic proteins inversely correlates with the rate of autophagy in yeast and mammalian cells, respectively, despite the fact that the routes of de novo AcCoA synthesis differ across phyla. Thus, we propose nucleo-cytosolic AcCoA to act as a conserved metabolic rheostat, linking the cellular metabolic state to the regulation of autophagy via effects on protein acetylation.     

The Caulobacter crescentus polar organelle development protein PodJ is differentially localized and is required for polar targeting of the PleC development regulator

Regulation of polar development and cell division in Caulobacter crescentus relies on the dynamic localization of several proteins to cell poles at specific stages of the cell cycle. The polar organelle development protein, PodJ, is required for the synthesis of the adhesive holdfast and pili. Here we show the cell cycle localization of PodJ and describe a novel role for this protein in controlling the dynamic localization of the developmental regulator PleC. In swarmer cells, a short form of PodJ is localized at the flagellated pole. Upon differentiation of the swarmer cell into a stalked cell, full length PodJ is synthesized and localizes to the pole opposite the stalk. In late predivisional cells, full length PodJ is processed into a short form which remains localized at the flagellar pole after cell division and is degraded during swarmer to stalked cell differentiation. Polar localization of the developmental regulator PleC requires the presence of PodJ. In contrast, the polar localization of PodJ is not dependent on the presence of PleC. These results indicate that PodJ is an important determinant for the localization of a major regulator of cell differentiation. Thus, PodJ acts directly or indirectly to target PleC to the incipient swarmer pole, to establish the cellular asymmetry that leads to the synthesis of holdfasts and pili at their proper subcellular location.