The bacterial Shiga toxin is composed of an enzymatically active A-subunit, and a receptor-binding homopentameric B-subunit (STxB) that mediates intracellular toxin trafficking. Upon STxB-mediated binding to the glycolipid globotriaosylceramide (Gb3) at the plasma membrane of target cells, Shiga toxin is internalized by clathrin-dependent and independent endocytosis. The formation of tubular membrane invaginations is an essential step in the clathrin-independent STxB uptake process. However, the mechanism by which STxB induces these invaginations has remained unclear. Using a combination of all-atom molecular dynamics and Monte Carlo simulations we show that the molecular architecture of STxB enables the following sequence of events: the Gb3 binding sites on STxB are arranged such that tight avidity-based binding results in a small increment of local curvature. Membrane-mediated clustering of several toxin molecules then creates a tubular membrane invagination that drives toxin entry into the cell. This mechanism requires: (1) a precise molecular architecture of the STxB binding sites; (2) a fluid bilayer in order for the tubular invagination to form. Although, STxB binding to the membrane requires specific interactions with Gb3 lipids, our study points to a generic molecular design principle for clathrin-independent endocytosis of nanoparticles.
There are more than 2 million bone grafting procedures performed annually in US alone. Despite significant efforts, the repair of large segmental bone defects is a substantial clinical challenge which...
Stress responses in both plants2؉ transient and survival of cultures subjected to NaCl stress was similar for the ACA2 transformant and K601. However, whereas K601 maintained low cytosolic Na ؉ predominantly by pumping it out across the plasma membrane, the transformant sequestered Na ؉ in internal organelles. This sequestration requires the presence of an endomembrane Na ؉ /H ؉ -antiporter, NHX1, which does not play a significant role in salt tolerance of wild type yeast except at acidic pH. Transcript levels of the plasma membrane Na ؉ -ATPase, ENA1, were strongly induced only in K601, whereas NHX1 was strongly induced in both K601 and the ACA2 transformant. The calmodulin kinase inhibitor KN62 significantly reduced the salt tolerance of the ACA2 transformant and the transcriptional induction of NHX1. Thus, the heterologous expression of a plant endomembrane Ca 2؉ pump results in the rapid depletion of cytosolic Ca 2؉ and the activation of an alternate mechanism for surviving saline stress.
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