Biomedical subjects
Publications and source records attributed to A Elgsaeter.
The presence of oblique striations on the cortex of the dense fibres of the rat spermatozoon is confirmed; they have a main periodicity of approximately 40 nm and a secondary one of about 20 nm. Disruption of spermatozoa by Triton X-100 and dithiothreitol and fixation in glutaraldehyde do not affect the pattern. The dense fibre cortex appears to consist of globular substructure units with a diameter of about 19 nm.
Rotary replication has been adapted to freeze-etching and evaluated using T4 polyheads, erythrocyte ghosts, and chloroplast membranes. Conventional electron microscopy, electron diffraction, and optical diffraction and filtering indicate that platinum-carbon rotary replication renders radially symmetrical contrast and 25 A resolution to freeze-etched specimens so as to clarify subunit structure not normally evident in unidirectional shadow replicas.
The low pH-induced aggregation of intramembrane particles in human erythrocyte membranes was studied in native membranes and in a reconstituted model system. A significant difference in such aggregation was found when samples of freshly prepared ghosts were compared to ghosts receiving pretreatments that removed most of spectrin-actin from underneath the membrane. All conditions effective in aggregating particles are equally effective in precipitating extracted mixtures of spectrin and actin. In Band 3-lipid recombinants, the pH-induced aggregation of particles was duplicated only in samples containing spectrin-actin that equilibrated with these recombinants in sucrose gradients. Therefore, it was proposed that spectrin-actin components, through their associations with the underlying intramembrane particles, could impede particle lateral mobility and also determine particle redistribution in erythrocyte membrane.
Physicochemical properties of mixtures of spectrin and actin extracted from human erythrocyte ghosts have been correlated with ultrastructural changes observed in freeze-fractured erythrocyte membranes. (1) Extracted mixtures of spectrin and actin have a very low solubility (less than 30 mug/ml) near their isoelectric point, pH 4.8. These mixtures are also precipitated by low concentrations of Ca2+, Mg2+, polylysine or basic proteins. (2) All conditions which precipitate extracts of spectrin and actin also induce aggregation of the intramembrane particles in spectrin-depleted erythrocyte ghosts. Precipitation of the residual spectrin molecules into small patches on the cytoplasmic surface of the ghost membrane is thought to be the cause of particle aggregations, implying an association between the spectrin molecules and the intramembrane particles. (3) When fresh ghosts are exposed to conditions which precipitate extracts of spectrin and actin, only limited particle aggregation occurs. Instead, the contraction of the intact spectrin meshwork induced by the precipitation conditions compresses the lipid bilayer of the membrane, causing it to bleb off particle-free, protein-free vesicles. (4) The absence of protein in these lipid vesicles implies that all the proteins of the erythrocyte membrane are immobilized by association with either the spectrin meshwork or the intramembrane particles.
We have used freeze-etching and SDS-polyacrylamide gel electrophoresis to study the conditions under which the intramembrane particles of the human erythrocyte ghost may be aggregated. The fibrous membrane protein, spectrin, can be almost entirely removed from erythrocyte ghosts with little or no change in the distribution of the particles. However, after spectrin depletion, particle aggregation in the plane of the membrane may be induced by conditions which cause little aggregation in freshly prepared ghosts. This suggests that the spectrin molecules form a molecular meshwork which limits the translational mobility of the erythrocyte membrane particles.