RESUMO
The most prominent structural feature of the parasitophorous vacuole (PV) in which the intracellular parasite Toxoplasma gondii proliferates is a membranous nanotubular network (MNN), which interconnects the parasites and the PV membrane. The MNN function remains unclear. The GRA2 and GRA6 proteins secreted from the parasite dense granules into the PV have been implicated in the MNN biogenesis. Amphipathic alpha-helices (AAHs) predicted in GRA2 and an alpha-helical hydrophobic domain predicted in GRA6 have been proposed to be responsible for their membrane association, thereby potentially molding the MMN in its structure. Here we report an analysis of the recombinant proteins (expressed in detergent-free conditions) by circular dichroism, which showed that full length GRA2 displays an alpha-helical secondary structure while recombinant GRA6 and GRA2 truncated of its AAHs are mainly random coiled. Dynamic light scattering and transmission electron microscopy showed that recombinant GRA6 and truncated GRA2 constitute a homogenous population of small particles (6-8 nm in diameter) while recombinant GRA2 corresponds to 2 populations of particles (â¼8-15 nm and up to 40 nm in diameter, respectively). The unusual properties of GRA2 due to its AAHs are discussed.
Assuntos
Antígenos de Protozoários/química , Proteínas de Protozoários/química , Antígenos de Protozoários/genética , Dicroísmo Circular , Luz , Microscopia Eletrônica de Transmissão , Dobramento de Proteína , Estrutura Secundária de Proteína , Proteínas de Protozoários/genética , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Espalhamento de Radiação , SolubilidadeRESUMO
Most Apicomplexa reside and multiply in the cytoplasm of their host cell, within a parasitophorous vacuole (PV) originating from both parasite and host cell components. Trafficking of parasite-encoded proteins destined to membrane compartments beyond the confine of the parasite plasma membrane is a process that offers a rich territory to explore novel mechanisms of protein-membrane interactions. Here, we focus on the PVs formed by the asexual stages of two pathogens of medical importance, Plasmodium and Toxoplasma. We compare the PVs of both parasites, with a particular emphasis on their evolutionary divergent compartmentalization within the host cell. We also discuss the existence of peculiar export mechanisms and/or sorting determinants that are potentially involved in the post-secretory targeting of parasite proteins to the PV subcompartments.
Assuntos
Apicomplexa/metabolismo , Apicomplexa/patogenicidade , Vacúolos/metabolismo , Animais , Apicomplexa/citologia , Membrana Celular/metabolismo , Interações Hospedeiro-Parasita , Plasmodium falciparum/metabolismo , Plasmodium falciparum/patogenicidade , Toxoplasma/metabolismo , Toxoplasma/patogenicidadeRESUMO
We study the spike statistics of neurons in a network with dynamically balanced excitation and inhibition. Our model, intended to represent a generic cortical column, comprises randomly connected excitatory and inhibitory leaky integrate-and-fire neurons, driven by excitatory input from an external population. The high connectivity permits a mean field description in which synaptic currents can be treated as gaussian noise, the mean and autocorrelation function of which are calculated self-consistently from the firing statistics of single model neurons. Within this description, a wide range of Fano factors is possible. We find that the irregularity of spike trains is controlled mainly by the strength of the synapses relative to the difference between the firing threshold and the postfiring reset level of the membrane potential. For moderately strong synapses, we find spike statistics very similar to those observed in primary visual cortex.
