SummaryToxoplasma gondii belongs to the phylum Apicomplexa and is an important cause of congenital disease and infection in immunocompromised patients. Like most apicomplexans, T. gondii possesses several plant-like features, such as the chloroplast-like organelle, the apicoplast. We describe and characterize a novel organelle in T. gondii tachyzoites, which is visible by light microscopy and possesses a broad similarity to the plant vacuole. Electron tomography shows the interaction of this vacuole with other organelles. The presence of a plant-like vacuolar proton pyrophosphatase (TgVP1), a vacuolar proton ATPase, a cathepsin L-like protease (TgCPL), an aquaporin (TgAQP1), as well as Ca 2+ /H + and Na + /H + exchange activities, supports similarity to the plant vacuole. Biochemical characterization of TgVP1 in enriched fractions shows a functional similarity to the respective plant enzyme. The organelle is a Ca 2+ store and appears to have protective effects against salt stress potentially linked to its sodium transport activity. In intracellular parasites, the organelle fragments, with some markers colocalizing with the late endosomal marker, Rab7, suggesting its involvement with the endocytic pathway. Studies on the characterization of this novel organelle will be relevant to the identification of novel targets for chemotherapy against T. gondii and other apicomplexan parasites as well.

Immature retrovirus particles contain radially arranged Gag polyproteins in which the N termini lie at the membrane and the C termini extend toward the particle's center. We related image features to the polyprotein domain structure by combining mutagenesis with cryoelectron microscopy and image analysis. The matrix (MA) domain appears as a thin layer tightly associated with the inner face of the viral membrane, separated from the capsid (CA) layer by a low-density region corresponding to its C terminus. Deletion of the entire p6 domain has no effect on the width or spacing of the density layers, suggesting that p6 is not ordered in immature human immunodeficiency virus type 1 (HIV-1). In vitro assembly of a recombinant Gag polyprotein containing only capsid (CA) and nucleocapsid (NC) domains results in the formation of nonenveloped spherical particles which display two layers with density matching that of the CA-NC portion of immature HIV-1 Gag particles. Authentic, immature HIV-1 displays additional surface features and an increased density between the lipid bilayers which reflect the presence of gp41. The other internal features match those of virus-like particles.

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Bacteriophage phi6 is an enveloped dsRNA virus with a segmented genome. Phi6 specifically packages one copy of each of its three genome segments into a preassembled polymerase complex. This leads to expansion of the polymerase complex, minus and plus strand RNA synthesis, and assembly of the nucleocapsid. The phi6 in vitro assembly and packaging system is a valuable model for dsRNA virus replication. The structure of the nucleocapsid at 7.5 A resolution presented here reveals the secondary structure of the two major capsid proteins. Asymmetric P1 dimers organize as an inner T = 1 shell, and P8 trimers organize as an outer T = 13 laevo shell. The organization of the P1 molecules in the unexpanded and expanded polymerase complex suggests that the expansion is accomplished by rigid body movements of the P1 monomers. This leads to exposure of new potential RNA binding surfaces to control the sequential packaging of the genome segments.

We have used (cryo) electron tomography to provide a 3-dimensional (3D) map of the intracellular membrane organization of human platelets at high spatial resolution. Our study shows that the open canalicular system and dense tubular system are highly intertwined and form close associations in specialized membrane regions. 3D reconstructions of individual ␣-granules revealed large heterogeneity in their membrane organization. On the basis of their divergent morphology, we categorized ␣-granules into the following subtypes: spherical granules with electron-dense and electron-lucent zone containing 12-nm von Willebrand factor tubules, subtypes containing a multitude of luminal vesicles, 50-nm-wide tubular organelles, and a population with 18.4-nm crystalline cross-striations. Low-dose (cryo) electron tomography and 3D reconstruction of whole vitrified platelets confirmed the existence of long tubular granules with a remarkably curved architecture. Immunoelectron microscopy confirmed that these extended structures represent ␣-granule subtypes. Tubular ␣-granules represent approximately 16% of the total ␣-granule population and are detected in approximately half of the platelet population. They express membrane-bound proteins GLUT3 and ␣IIb-␤3 integrin and contain abundant fibrinogen and albumin but low levels of ␤-thromboglobulin and no von Willebrand factor. Our 3D study demonstrates that, besides the existence of morphologically different ␣-granule subtypes, high spatial segregation of cargo exists within individual ␣-granules. (Blood. 2010;116(7):1147-1156) IntroductionBlood platelets are the smallest cells in our circulation. They play a central role in the arrest of bleeding after damage of a blood vessel and are crucial elements in the development of thrombosis. 1,2 On injury, platelets rapidly adhere to components of the subendothelium, followed by shape change and subsequent granule secretion. 3 These rapid membrane dynamics are crucial for the progression of platelet-substrate interaction (spreading) and subsequent plateletplatelet interaction (aggregation), ultimately leading to the formation of a platelet plug and the arrest of bleeding. 4 Platelets contain several distinct membrane systems: (1) the open canalicular system (OCS), which is continuous with the cell surface and serves as a membrane reservoir during shape change and spreading 5,6 ; (2) the dense tubular system (DTS), representing the platelet smooth endoplasmic reticulum 7 ; and (3) secretory organelles. Four types of secretory organelles have been identified in platelets, based on their ultrastructure and selective protein composition: ␣-granules, dense granules, multivesicular bodies, and lysosomes. 8-10 ␣-Granules are the major secretory organelles and appear in electron microscopy cross sections as 200-to 500-nm spherical organelles. Platelet ␣-granules and dense granules are differentially released and play crucial roles in the secondary platelet response. 11,12 Recent studies have suggested the existence of ␣-granule subclasses with dif...