Publications (41) View all
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Article: The holo-apoptosome: activation of procaspase-9 and interactions with caspase-3.
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ABSTRACT: Activation of procaspase-9 on the apoptosome is a pivotal step in the intrinsic cell death pathway. We now provide further evidence that caspase recruitment domains of pc-9 and Apaf-1 form a CARD-CARD disk that is flexibly tethered to the apoptosome. In addition, a 3D reconstruction of the pc-9 apoptosome was calculated without symmetry restraints. In this structure, p20 and p10 catalytic domains of a single pc-9 interact with nucleotide binding domains of adjacent Apaf-1 subunits. Together, disk assembly and pc-9 binding create an asymmetric proteolysis machine. We also show that CARD-p20 and p20-p10 linkers play important roles in pc-9 activation. Based on the data, we propose a proximity-induced association model for pc-9 activation on the apoptosome. We also show that pc-9 and caspase-3 have overlapping binding sites on the central hub. These binding sites may play a role in pc-3 activation and could allow the formation of hybrid apoptosomes with pc-9 and caspase-3 proteolytic activities.Structure 08/2011; 19(8):1084-96. · 6.35 Impact Factor -
Article: Streptolysin O clearance through sequestration into blebs that bud passively from the plasma membrane.
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ABSTRACT: Cells survive exposure to bacterial pore-forming toxins, such as streptolysin O (SLO), through mechanisms that remain unclear. Previous studies have suggested that these toxins are cleared by endocytosis. However, the experiments reported here failed to reveal any evidence for endocytosis of SLO, nor did they reveal any signs of damage to endosomal membranes predicted from such endocytosis. Instead, we illustrate that SLO induces a characteristic form of plasma membrane blebbing that allows cells to shed SLO by the process known as ectocytosis. Specifically, 'deep-etch' electron microscopy of cells exposed to SLO illustrates that the toxin is rapidly sequestered into domains in the plasmalemma greatly enriched in SLO pores, and these domains bleb outwards and bud from the cell surface into the medium. Such ectocytosis is even observed in cells that have been chemically fixed before exposure to SLO, suggesting that it is caused by a direct physical action of the toxin on the cell membrane, rather than by an active cellular reaction. We conclude, therefore, that ectocytosis is an important means for SLO clearance and hypothesize that this is a primary method by which cells defend themselves generally against pore-forming toxins.Journal of Cell Science 07/2011; 124(Pt 14):2414-23. · 6.11 Impact Factor -
Article: The origins and evolution of freeze-etch electron microscopy.
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ABSTRACT: The introduction of the Balzers freeze-fracture machine by Moor in 1961 had a much greater impact on the advancement of electron microscopy than he could have imagined. Devised originally to circumvent the dangers of classical thin-section techniques, as well as to provide unique en face views of cell membranes, freeze-fracturing proved to be crucial for developing modern concepts of how biological membranes are organized and proved that membranes are bilayers of lipids within which proteins float and self-assemble. Later, when freeze-fracturing was combined with methods for freezing cells that avoided the fixation and cryoprotection steps that Moor still had to use to prepare the samples for his original invention, it became a means for capturing membrane dynamics on the millisecond time-scale, thus allowing a deeper understanding of the functions of biological membranes in living cells as well as their static ultrastructure. Finally, the realization that unfixed, non-cryoprotected samples could be deeply vacuum-etched or even freeze-dried after freeze-fracturing opened up a whole new way to image all the other molecular components of cells besides their membranes and also provided a powerful means to image the interactions of all the cytoplasmic components with the various membranes of the cell. The purpose of this review is to outline the history of these technical developments, to describe how they are being used in electron microscopy today and to suggest how they can be improved in order to further their utility for biological electron microscopy in the future.Journal of electron microscopy 01/2011; 60 Suppl 1:S3-29. · 1.31 Impact Factor -
Article: Molecularly targeted nanocarriers deliver the cytolytic peptide melittin specifically to tumor cells in mice, reducing tumor growth.
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ABSTRACT: The in vivo application of cytolytic peptides for cancer therapeutics is hampered by toxicity, nonspecificity, and degradation. We previously developed a specific strategy to synthesize a nanoscale delivery vehicle for cytolytic peptides by incorporating the nonspecific amphipathic cytolytic peptide melittin into the outer lipid monolayer of a perfluorocarbon nanoparticle. Here, we have demonstrated that the favorable pharmacokinetics of this nanocarrier allows accumulation of melittin in murine tumors in vivo and a dramatic reduction in tumor growth without any apparent signs of toxicity. Furthermore, direct assays demonstrated that molecularly targeted nanocarriers selectively delivered melittin to multiple tumor targets, including endothelial and cancer cells, through a hemifusion mechanism. In cells, this hemifusion and transfer process did not disrupt the surface membrane but did trigger apoptosis and in animals caused regression of precancerous dysplastic lesions. Collectively, these data suggest that the ability to restrain the wide-spectrum lytic potential of a potent cytolytic peptide in a nanovehicle, combined with the flexibility of passive or active molecular targeting, represents an innovative molecular design for chemotherapy with broad-spectrum cytolytic peptides for the treatment of cancer at multiple stages.The Journal of clinical investigation 10/2009; 119(9):2830-42. · 15.39 Impact Factor -
Article: Coordinate regulation of glycan degradation and polysaccharide capsule biosynthesis by a prominent human gut symbiont.
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ABSTRACT: Bacteria in the distal human gut have evolved diverse abilities to metabolize complex glycans, including the capacity to degrade these compounds as nutrients and to assemble their component sugars into new polymers such as extracellular capsules. The human gut bacterium Bacteroides thetaiotaomicron is well endowed with the ability to metabolize both host- and diet-derived glycans. Its genome contains 88 different polysaccharide utilization loci (PULs) for complex glycan catabolism and eight different gene clusters for capsular polysaccharide biosynthesis. Here, we investigate one of the prominent mechanisms by which this gut symbiont regulates many PULs involved in host mucin O-glycan degradation; namely, transcriptional regulation via the concerted interactions of cell-envelope-localized TonB-dependent transporters, extra-cytoplasmic function sigma factors and anti-sigma factors, which participate together in a regulatory pathway termed trans-envelope signaling. Unexpectedly, we found that several different trans-envelope signaling switches involved in PUL-mediated O-glycan degradation also modulate capsular polysaccharide synthesis. A novel regulatory pathway, which is dependent on expression of O-glycan-targeting outer membrane proteins, governs this coordinated regulation of glycan catabolism and capsule synthesis. This latter finding provides a new link in the dynamic interplay between complex glycan metabolism, microbial physiology, and host responses that occurs during colonization of the gut.Journal of Biological Chemistry 05/2009; 284(27):18445-57. · 4.77 Impact Factor
