Reconstitution of cytochrome f/b6 and CF0-CF1 ATP synthetase complexes into phospholipid and galactolipid liposomes

Cytochrome f/b6 and ATP synthetase (CF0-CF1) complexes from spinach chloroplasts have been reconstituted into liposomes prepared from soybean phospholipids and purified spinach galactolipids. Freeze- fracture analysis revealed homogeneous populations of particles spanning the lipid bilayers with their elongated axes perpendicular to the membrane plane. The lipid composition of the liposomes had no effect on the size of the reconstituted complexes, the average diameter of cytochrome f/b6 complex measuring 8.5 nm, and of the CF0 base piece of the ATP synthetase, 9.5 nm. When reconstituted cytochrome f/b6 complexes were cross-linked by means of antibodies prepared against the whole complex, the thus aggregated particles formed either hexagonal or square arrays. In both instances the center-to-center spacing of the particles was 8.3 nm, thereby suggesting that this value could be closer to the real diameter of the complexes than the one obtained from measuring individual particles. Assuming an ellipsoidal shape for these particles, and using a measured height of 11 nm, a molecular weight of approximately 280,000 could be calculated for the reconstituted cytochrome f/b6 complex, consistent with a dimeric configuration. In many instances the crystalline sheets of antibody-aggregated cytochrome f/b6 complexes were found to be free in the buffer solution; apparently the antibody-induced strains caused the sheet-like aggregates to pop out of the liposomal membranes. Agglutination studies of inside-out and right-side-out thylakoid vesicles revealed the antigenic determinants of the cytochrome f and cytochrome b6 polypeptides to be exposed on the inner thylakoid surface and to be present in stacked and unstacked membrane regions. The molecular weight calculated from the size of freeze-fractured CF0 base pieces was over twice the value determined by x-ray scattering data. This discrepancy may be caused by significant lipid domains within the base piece, or by an unusual fracturing behavior of the base piece in reconstituted liposomes.     

THE ULTRASTRUCTURE OF SCENEDESMUS (CHLOROPHYCEAE). II. CELL DIVISION AND COLONY FORMATION1

Cell division in Scenedesmus is fairly typical of other chlorococcalean genera. The closed spindle has centrioles at polar fenestrae and apparently a series of nuclear divisions precedes cytokinesis. The phycoplast system of cytokinetic microtubules predicts the path of cleavage furrows whose mode of formation is obscure. Before and during cell division, the endoplasmic reticulum invariably accumulates granular material which later, during cytokinesis, appears to he secreted via the golgi bodies. Similar dense granular material then at accumulates outside the forming daughter cells but inside the parental wall, as the latter begins eroding away. By the end of colony formation, the cellulosic parental wall has disappeared, leaving its outer sheath and attached ornamentative features (spines, combs, reticulate or warty layer, etc.) intact as a “ghost.” The spines and combs of new colonies appear to condense out of the extracellular aggregate; their precise mode of formation is obscure. As they form, the daughter cells, having become rearranged within the parental wall, stick to one another apparently at specific sites on their outer surface. A trilaminar (sporopollenin-containing) layer arises first in each cell at these adhesive sites and immediately afterwards, dense material aggregates between the adjacent layers to give rise to the coenobial adhesive. Plaques of the trilaminar layer later appear over the rest of the cell's surface; they grow and fuse so that eventually each cell is enclosed by one continuous Trilaminar Sheath (TLS). While the plaques are forming, another dense layer materializes around the whole coenobium. Depending on the species, this layer turns into either the warty layer, in which instance it is applied directly on to the surface of the TLS except near the coenobial adhesive, or else it becomes the reticulate layer, in which instance it remains entirely separate from the TLS, soon acquiring the complex system of propping spikelets which suspend it from the coenobial surface. When fully farmed, the daughter coenobium is tightly compressed within the parental TLS, with its spines folded lengthwise along the daughter cells. Release of the colony follows a quite explosive rupturing of the parental TLS, and immediately upon release, the daughter colony flattens out and erects its spines.     

Orientation of Membrane Vesicles from Escherichia coli as Detected by Freeze-Cleave Electron Microscopy

The application of freeze-cleave electron microscopy to whole cells of Escherichia coli revealed that the particles exposed on the resulting two inner membrane faces are asymmetrically distributed. This method can therefore be used to determine the orientation of membrane vesicles from E. coli. Membrane vesicles freshly prepared in potassium phosphate buffer (K(+)-vesicles) by osmotic lysis of spheroplasts consisted almost entirely of right-side-out vesicles. Their size suggested that each cell gives rise to one vesicle. When the membrane vesicles were subjected to one cycle of freezing and thawing, the number of inside-out vesicles rose to about 25%. However, due to the small size of most of the inside-out vesicles, these contribute only 2 to 3% of the total membrane surface area of the preparation. The inside-out vesicles appear to arise from infoldings of the membrane of right-side-out vesicles. They also accumulate within the latter, thus producing multivesicular membrane sacs. Na(+)-vesicles (vesicles prepared in sodium phosphate buffer) subjected to freezing and thawing appeared to lose structural rigidity more than did K(+)-vesicles. In contrast to the membrane vesicles prepared by the osmotic lysis of spheroplasts, those obtained by breaking intact cells by a single passage through a French pressure cell were uniformly very small (only 40 to 110 nm in diameter); approximately 60 to 80% were inside-out. To reconcile the polarity of the membrane vesicles with the enzymic activities of such preparations, we propose that "dislocation" of membrane proteins occurs during osmotic lysis of spheroplasts.