The separation of microsomal membranes and free polyribosomes by gel filtration

Microsomal membrane subfractionation by a lectin affinity method

Concanavalin A-agarose treatment of rat liver post-mitochondrial supernatant removes a fraction rich in cholesterol and 5'-nucleotidase activity but low in glucose-6-phosphatase. At the same time, radiolabel associated with the cell surface is removed. We interpret these findings as evidence that concanavalin A binds to, and under these circumstances will remove, fragments of plasma membrane present in the microsomal fraction and believe that this may be of use in the gentle, and rapid subfractionation of microsomal membranes.

Preparation and properties of a complex from rat liver of polyribosomes with components of the cytoskeleton

Gel filtration with 1% agarose (Bio-Gel A-150m) separates polyribosomes bound to microsomal membranes from 'free' polyribosomes when these fractions are prepared by standard centrifugal techniques. However, when polyribosomes contained in an unfractionated postmitochondrial supernatant are run on an identical column, over 90% of the total polyribosomes are present as aggregates, designated 'membrane-cytomatrix', which are eluted in the column void volume. Polyribosomes are not released from these aggregates on removal of microsomal phospholipids by treatment of postmitochondrial supernatant with 1% Triton X-100, a neutral detergent. The aggregates are disrupted by the usual ultracentrifugation techniques used in subcellular fractionation. After treatment of membrane-cytomatrix with Triton X-100 to remove phospholipids and membrane proteins, 58% of the polyribosomes still remain associated with protein-containing complexes in the form of a cytomatrix and are not 'free'. Preparations of both membrane-cytomatrix and cytomatrix are capable of sustained protein synthesis. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed that the cytoskeletal proteins actin and myosin are present in the cytomatrix. Incubation of cytomatrix preparations with the actin-depolymerizing agent deoxyribonuclease I caused release of the polyribosomes. Polyribosome release by deoxyribonuclease I was prevented by prior incubation with phalloidin, which is known to stabilize F-actin. Thus polyribosomes are associated with cytoskeletal elements in rat liver, and this association is dependent on polymeric forms of actin.