Structural basis for Chk1 inhibition by UCN-01

Chk1 is a serine-threonine kinase that plays an important role in the DNA damage response, including G(2)/M cell cycle control. UCN-01 (7-hydroxystaurosporine), currently in clinical trials, has recently been shown to be a potent Chk1 inhibitor that abrogates the G(2)/M checkpoint induced by DNA-damaging agents. To understand the structural basis of Chk1 inhibition by UCN-01, we determined the crystal structure of the Chk1 kinase domain in complex with UCN-01. Chk1 structures with staurosporine and its analog SB-218078 were also determined. All three compounds bind in the ATP-binding pocket of Chk1, producing only slight changes in the protein conformation. Selectivity of UCN-01 toward Chk1 over cyclin-dependent kinases can be explained by the presence of a hydroxyl group in the lactam moiety interacting with the ATP-binding pocket. Hydrophobic interactions and hydrogen-bonding interactions were observed in the structures between UCN-01 and the Chk1 kinase domain. The high structural complementarity of these interactions is consistent with the potency and selectivity of UCN-01.     

Hyposialylation of integrins stimulates the activity of myeloid fibronectin receptors

Despite numerous reports suggesting that beta(1) integrin receptors undergo differential glycosylation, the potential role of N-linked carbohydrates in modulating integrin function has been largely ignored. In the present study, we find that beta(1) integrins are differentially glycosylated during phorbol ester (PMA)-stimulated differentiation of myeloid cells along the monocyte/macrophage lineage. PMA treatment of two myeloid cell lines, U937 and THP-1, induces a down-regulation in expression of the ST6Gal I sialyltransferase. Correspondingly, the beta(1) integrin subunit becomes hyposialylated, suggesting that the beta(1) integrin is a substrate for this enzyme. The expression of hyposialylated beta(1) integrin isoforms is temporally correlated with enhanced binding of myeloid cells to fibronectin, and, importantly, fibronectin binding is inhibited when the Golgi disrupter, brefeldin A, is used to block the expression of the hyposialylated form. Consistent with the observation that cells with hyposialylated integrins are more adhesive to fibronectin, we demonstrate that the enzymatic removal of sialic acid residues from purified alpha(5)beta(1) integrins stimulates fibronectin binding by these integrins. These data support the hypothesis that unsialylated beta(1) integrins are more adhesive to fibronectin, although desialylation of alpha(5) subunits could also contribute to increased fibronectin binding. Collectively our results suggest a novel mechanism for regulation of the beta(1) integrin family of cell adhesion receptors.     

Charged amino acids in the sixth transmembrane helix of multidrug resistance protein 1 (MRP1/ABCC1) are critical determinants of transport activity

The multidrug resistance protein, MRP1 (ABCC1), is an ATP-binding cassette transporter that confers resistance to chemotherapeutic agents. MRP1 also mediates transport of organic anions such as leukotriene C(4) (LTC(4)), 17beta-estradiol 17-(beta-d-glucuronide) (E(2)17betaG), estrone 3-sulfate, methotrexate (MTX), and GSH. We replaced three charged amino acids, Lys(332), His(335), and Asp(336), predicted to be in the sixth transmembrane (TM6) helix of MRP1 with neutral and oppositely charged amino acids and determined the effect on substrate specificity and transport activity. All mutants were expressed in transfected human embryonic kidney cells at levels comparable with wild-type MRP1, and confocal microscopy showed that they were correctly routed to the plasma membrane. Vesicular transport studies revealed that the MRP1-Lys(332) mutants had lost the ability to transport LTC(4), and GSH transport was reduced; whereas E(2)17betaG, estrone 3-sulfate, and MTX transport were unaffected. E(2)17betaG transport was not inhibited by LTC(4) and could not be photolabeled with [(3)H]LTC(4), indicating that the MRP1-Lys(332) mutants no longer bound this substrate. Substitutions of MRP1-His(335) also selectively diminished LTC(4) transport and photolabeling but to a lesser extent. Kinetic analyses showed that V(max) (LTC(4)) of these mutants was decreased but K(m) was unchanged. In contrast to the selective loss of LTC(4) transport in the Lys(332) and His(335) mutants, the MRP1-Asp(336) mutants no longer transported LTC(4), E(2)17betaG, estrone 3-sulfate, or GSH, and transport of MTX was reduced by >50%. Lys(332), His(335), and Asp(336) of TM6 are predicted to be in the outer leaflet of the membrane and are all capable of forming intrahelical and interhelical ion pairs and hydrogen bonds. The importance of Lys(332) and His(335) in determining substrate specificity and of Asp(336) in overall transport activity suggests that such interactions are critical for the binding and transport of LTC(4) and other substrates of MRP1.     

Connecting replication and recombination

Replication forks frequently break and must be repaired by recombination. A reconstituted reaction now allows the factors that coordinate conversion from a recombination intermediate back to a replication fork to be defined. The PriA protein plays a key role in this control.     

Fission yeast COP9/signalosome suppresses cullin activity through recruitment of the deubiquitylating enzyme Ubp12p

The COP9/signalosome (CSN) is known to remove the stimulatory NEDD8 modification from cullins. The activity of the fission yeast cullins Pcu1p and Pcu3p is dramatically stimulated when retrieved from csn mutants but inhibited by purified CSN. This inhibition is independent of cullin deneddylation but mediated by the CSN-associated deubiquitylating enzyme Ubp12p, which forms a complex with Pcu3p in a CSN-dependent manner. In ubp12 mutants, as in csn mutants, Pcu3p activity is stimulated. CSN is required for efficient targeting of Ubp12p to the nucleus, where both cullins reside. Finally, the CSN/Ubp12p pathway maintains the stability of the Pcu1p-associated substrate-specific adaptor protein Pop1p. We propose that CSN/Ubp12p-mediated deubiquitylation creates an environment for the safe de novo assembly of cullin complexes by counteracting the autocatalytic destruction of adaptor proteins.     

Lytic enzyme activity in peat is increased by substrate amendment with chitin: Implications for the control of<Emphasis Type="Italic">Phytophthora fragariae</Emphasis> in<Emphasis Type="Italic">Fragaria vesca</Emphasis>

Chitin is reported to stimulate mycorrhizas and antagonistic, lytic-enzyme producing soil microorganisms. Here mycorrhizal and non-mycorrhizal strawberry (Fragaria vesca) plants were grown in peat growth-substrate and in peat amended with chitin (Suppressor™). Chitinase and cellulase activity was determined and the plants were challenged by inoculation withPhytophthora fragariae, the causal agent of redcore disease. Disease resistance was only found in the mycorrhizal strawberry-chitin amendment treatment and depended on the duration of the mycorrhiza’s growth in the amended substrate before potting on to a chitin-free growth substrate. The practical application of chitin substrate amendment in the exploitation of mycorrhizal biological control is discussed.     

Regulation of the generation and maintenance of T-cell memory: a direct,default pathway from effectors to memory cells

Memory T cells are derived directly from effector cells without need for additional antigen, TcR triggering or induced cytokines. A large fraction of effectors can become memory cells without division, supporting a default pathway with little further differentiation. This suggests that the same signals during infection/vaccination determine the extent and nature of both effector and memory cell development.