We continued our work on the membrane biology of pathogenic processes. Membrane fusion intermediates induced by the glycosylphosphatidylinositol-linked ectodomain of influenza hemagglutinin (GPI-HA) were investigated by rapidly freeze, freeze-substitution, thin section electron microscopy, and with simultaneous recordings of whole-cell admittance and fluorescence. Upon triggering, the previously separated membranes developed, when viewed by electron microscopy, numerous hourglass shaped points of membrane contact sites (~10-130 nm waist). Stereo pairs showed close membrane contact at peaks of complementary protrusions, arising from each membrane. With HA, there were fewer contacts, with wide fusion pores. Physiological measurements showed fast lipid dye mixing between cells after acidification, and either fusion pore formation or the lack thereof (true hemifusion). For the earliest pores a similar conductance distribution and frequency of flickering pores were detected for both HA and GPI-HA. For GPI-HA, lipid mixing was detected prior to, during, or after pore opening, whereas for HA lipid mixing is seen only after pore opening. Our findings are consistent with a pathway wherein conformational changes in the ectodomain of HA pulls membranes towards each other to form a contact site, then hemifusion and pore formation initiate in a small percentage of these contact sites. Finally, the transmembrane domain of HA is needed to complete membrane fusion for macromolecular content mixing. Growth of the malaria parasite in human red blood cells (RBCs) is accompanied by an increased uptake of many solutes including anions, sugars, purines, amino acids and organic cations. Although the pharmacological properties and selectivity of this uptake suggest that a chloride channel is involved, the precise mechanism has not been identified. Moreover, the location of this uptake in the infected RBC is unknown because tracer studies are complicated by possible uptake through fluid-phase pinocytosis or membranous ducts. In this project, we have studied the permeability of infected RBCs using the whole-cell voltage-clamp method, never before performed on infected cells. With this method, uninfected RBCs had ohmic whole-cell conductances of less than 100 pS, consistent with their low tracer permeabilities. In contrast, trophozoite-infected RBCs exhibited voltage-dependent, non-saturating currents that were 150-fold larger, predominantly carried by anions and abruptly abolished by channel blockers. Patch-clamp measurements and spectral analysis confirmed that a small (< 10 pS) ion channel on the infected RBC surface, present at about 1,000 copies per cell, is responsible for these currents. Because its pharmacological properties and substrate selectivities match those seen with tracer studies, this channel accounts for the increased uptake of small solutes in infected RBCs. The surface location of this new channel and its permeability to organic solutes needed for parasite growth indicate that it may have a primary role in a sequential diffusive pathway for parasite nutrient acquisition. This channel may be blocked, and so it is an attractive target for drug development.
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