Role of the [65-72] disulfide bond in oxidative folding of bovine pancreatic ribonuclease A

To assess the role of the [65-72] disulfide bond in the oxidative folding of RNase A, use has been made of [C65S, C72S], a three-disulfide-containing mutant of RNase A which regenerates from its two-disulfide precursor in an oxidation and conformational folding-coupled rate-determining step. The distribution of disulfide bonds in the one-disulfide-containing ensemble of this mutant has been characterized. In general, the disulfide-bond distribution in its 1S ensemble agrees relatively well with the corresponding distribution in wt-RNase A and with distributions based on calculations of loop entropy, except for the absence of the [65-72] disulfide bond. There is no bias (over the entropic influence) for the three native disulfide bonds, [26-84], [40-95], and [58-110]. Previous oxidative folding results for wt-RNase A indicated the predominance of the des [40-95] intermediate over des [65-72] after the rate-determining step in the regeneration process. Considering that there is no preferential distribution of disulfides in the 1S ensemble of [C65S, C72S], in contrast to the preferential population of the [65-72] disulfide bond in wt-RNase A, these results indicate a critical role for the [65-72] disulfide bond in the regeneration of wt-RNase A. Furthermore, analysis of the disulfide distribution of the 1S intermediates of [C65S, C72S] compared to that of wt-RNase A lends support for a physicochemical basis for the previously observed slow folding rate of this mutant, compared to its analogue (des [65-72]) of wt-RNase A.     

Active lipoprotein precursors in the Gram-positive eubacterium Lactococcus lactis

Lipid-modified proteins play important roles at the interface between eubacterial cells and their environment. The importance of lipoprotein processing by signal peptidase II (SPase II) is underscored by the fact that this enzyme is essential for viability of the Gram-negative eubacterium Escherichia coli. In contrast, SPase II is not essential for growth and viability of the Gram-positive eubacterium Bacillus subtilis. This could be due to alternative amino-terminal lipoprotein processing, which was shown previously to occur in SPase II mutants of B. subtilis. Alternatively, uncleaved lipoprotein precursors might be functional. To explore further the importance of lipoprotein processing in Gram-positive eubacteria, an SPase II mutant strain of Lactococcus lactis was constructed. Although some of the 39 (predicted) lactococcal lipoproteins, such as PrtM and OppA, are essential for growth in milk, the growth of SPase II mutant L. lactis cells in this medium was not affected. Furthermore, the activity of the strictly PrtM-dependent extracellular protease PrtP, which is required for casein degradation, was not impaired in the absence of SPase II. Importantly, no alternative processing of pre-PrtM and pre-OppA was observed in cells lacking SPase II. Taken together, these findings show for the first time that authentic lipoprotein precursors retain biological activity.     

Stimulation of phosphatidylserine biosynthesis and facilitation of UV-induced apoptosis in Chinese hamster ovary cells overexpressing phospholipid scramblase 1

Members of the phospholipid scramblase (PLSCR) family play active roles in altering lipid asymmetry at the plasma membrane including phosphatidylserine (PtdSer) exposure on the cell surface. To determine whether PtdSer biosynthesis and externalization are altered by PLSCR activities during apoptosis, Chinese hamster ovary K1 cell lines stably overexpressing PLSCR1 and PLSCR2 were established. PLSCR1 was localized on the plasma membrane, whereas PLSCR2 was predominantly in the nucleus. Cells overexpressing PLSCR1 showed suppressed growth, altered cell morphology, and higher basal levels of cell death. Following UV irradiation, these cells showed earlier and enhanced PtdSer exposure, increased caspase-3 activation, apoptotic nuclear changes, and PARP cleavage indicative of apoptosis. UV irradiation in cells overexpressing PLSCR1 led to a 4-fold stimulation of PtdSer synthesis (accompanied by increased movement of newly made PtdSer into microvesicles) relative to untreated PLSCR1 cells, whereas PtdSer formation in UV-irradiated vector control cells increased only by 2-fold. No differences in these responses were observed between PLSCR2-expressing cells and vector controls. PtdSer synthesis and its transbilayer movement stimulated by PLSCR1 overexpression were blocked by a caspase inhibitor along with progression of apoptosis. Thus, our studies showed that overexpression of PLSCR1 in Chinese hamster ovary K1 cells stimulated caspase-dependent PtdSer externalization and synthesis, implying an up-regulation of PtdSer formation in response to enhanced outward movement of this phospholipid to the cell surface during apoptosis. PLSCR1 also appears to influence progression of UV-induced apoptosis and could be a point of regulation or intervention during programmed cell death.     

Standardization of methods reduces variability: explanation for historical discrepancies in biochemical screening

Over the past decade, many studies have argued the relative merits of different chromosome abnormality biochemical screening protocols in different labs. Results and interpretations have varied markedly (e.g., double vs. triple screening). In this study we sought to compare coefficients of variation (CV) among 12 laboratories in one system, using identical and different methodologies for the three parameters. Ten identical specimens were processed as part of the 1999 College of American Pathologists (CAP) (FP-A, FP-B) proficiency tests. Results were compared among 12 laboratories using the Abbott ELISA for alpha-fetoprotein (AFP) and human chorionic gonadotropin (hCG), and two methods for estriol [Diagnostic Services Laboratory (DSL) and an "in house" assay]. The range on the 10 specimens of means for AFP varied from 12.56 to 117.87; hCG 14.05-68.08; and estriol 0.61-2.73. CV for AFP specimens averaged 10%, hCG 8%, and estriol 35% (F = 22.4). However, when only DSL was used for estriol, the CV was reduced to 8.7%. Standardization of AFP and hCG across 12 labs has reduced CV to <10%, which is similar to accepted between run results. Wide variation of uE3 among different methods may explain the widely divergent results in the literature. With national standardization of all parameters, the widely divergent results seen in the literature should narrow, and regional medians will no longer be necessary.     

Expression of a unique drug-resistant Hsp90 ortholog by the nematode Caenorhabditis elegans

In all species studied to date, the function of heat shock protein 90 (Hsp90), a ubiquitous and evolutionarily conserved molecular chaperone, is inhibited selectively by the natural product drugs geldanamycin (GA) and radicicol. Crystal structures of the N-terminal region of yeast and human Hsp90 have revealed that these compounds interact with the chaperone in a Bergerat-type adenine nucleotide-binding fold shared throughout the gyrase, Hsp90, histidine kinase mutL (GHKL) superfamily of adenosine triphosphatases. To better understand the consequences of disrupting Hsp90 function in a genetically tractable multicellular organism, we exposed the soil-dwelling nematode Caenorhabditis elegans to GA under a variety of conditions designed to optimize drug uptake. Mutations in the gene encoding C elegans Hsp90 affect larval viability, dauer development, fertility, and life span. However, exposure of worms to GA produced no discernable phenotypes, although the amino acid sequence of worm Hsp90 is 85% homologous to that of human Hsp90. Consistent with this observation, we found that solid phase-immobilized GA failed to bind worm Hsp90 from worm protein extracts or when translated in a rabbit reticulocyte lysate system. Further, affinity precipitation studies using chimeric worm-vertebrate fusion proteins or worm C-terminal truncations expressed in reticulocyte lysate revealed that the conserved nucleotide-binding fold of worm Hsp90 exhibits the novel ability to bind adenosine triphosphate but not GA. Despite its unusual GA resistance, worm Hsp90 appeared fully functional when expressed in a vertebrate background. It heterodimerized with its vertebrate counterpart and showed no evidence of compromising its essential cellular functions. Heterologous expression of worm Hsp90 in tumor cells, however, did not render them GA resistant. These findings provide new insights into the nature of unusual N-terminal nucleotide-binding fold of Hsp90 and suggest that target-related drug resistance is unlikely to emerge in patients receiving GA-like chemotherapeutic agents.     

Rapamycin potentiates transforming growth factor beta-induced growth arrest in nontransformed,oncogene-transformed,and human cancer cells

Transforming growth factor beta (TGF-beta) induces cell cycle arrest of most nontransformed epithelial cell lines. In contrast, many human carcinomas are refractory to the growth-inhibitory effect of TGF-beta. TGF-beta overexpression inhibits tumorigenesis, and abolition of TGF-beta signaling accelerates tumorigenesis, suggesting that TGF-beta acts as a tumor suppressor in mouse models of cancer. A screen to identify agents that potentiate TGF-beta-induced growth arrest demonstrated that the potential anticancer agent rapamycin cooperated with TGF-beta to induce growth arrest in multiple cell lines. Rapamycin also augmented the ability of TGF-beta to inhibit the proliferation of E2F1-, c-Myc-, and (V12)H-Ras-transformed cells, even though these cells were insensitive to TGF-beta-mediated growth arrest in the absence of rapamycin. Rapamycin potentiation of TGF-beta-induced growth arrest could not be explained by increases in TGF-beta receptor levels or rapamycin-induced dissociation of FKBP12 from the TGF-beta type I receptor. Significantly, TGF-beta and rapamycin cooperated to induce growth inhibition of human carcinoma cells that are resistant to TGF-beta-induced growth arrest, and arrest correlated with a suppression of Cdk2 kinase activity. Inhibition of Cdk2 activity was associated with increased binding of p21 and p27 to Cdk2 and decreased phosphorylation of Cdk2 on Thr(160). Increased p21 and p27 binding to Cdk2 was accompanied by decreased p130, p107, and E2F4 binding to Cdk2. Together, these results indicate that rapamycin and TGF-beta cooperate to inhibit the proliferation of nontransformed cells and cancer cells by acting in concert to inhibit Cdk2 activity.     

Measuring real levels of oxygen in vivo: opportunities and challenges

Molecular oxygen has multiple roles in cells, which can have different dependencies on the amount of oxygen at particular sites. Therefore, in order to relate oxygen to the processes under consideration, one must know the amount of oxygen present at the site of interest. The use of the various techniques generally is complementary rather than competitive. The key to their successful use requires an understanding of the capabilities of the techniques and of the nature of the process that is being investigated. This paper provides a framework for obtaining such an understanding.