A B S T R A C TIn a medium of high ionic strength, rat liver rough microsomes can be nondestructively disassembled into ribosomes and stripped membranes if nascent polypeptides are discharged from the bound ribosomes by reaction with puromycin. At 750 mM KC1, 5 mM MgCl~, 50 m2~ Tris.HC1, pH 7 5, up to 85% of all bound ribosomes are released from the membranes after incubation at room temperature with 1 mM puromycin. The ribosomes are released as subunits which are active in peptide synthesis if programmed with polyuridylic acid. The ribosome-denuded, or stripped, rough microsomes (RM) can be recovered as intact, essentially unaltered membranous vesicles Judging from the incorporation of [SH]puromycin into hot acid-insoluble material and from the release of [SH]leucine-labeled nascent polypeptide chains from bound ribosomes, puromycin coupling occurs almost as well at low (25-100 rnM) as at high (500-1000 raM) KC1 concentrations. Since puromycin-dependent ribosome release only occurs at high ionic strength, it appears that ribosomes are bound to membranes via two types of interactions: a direct one between the membrane and the large ribosomal subunit (labile at high KC1 concentration) and an indirect one in which the nascent chain anchors the ribosome to the membrane (puromycin labile). The nascent chains of ribosomes specifically released by puromycin remain tightly associated with the stripped membranes. Some membrane-bound ribosomes (up to 40%) can be nondestructively released in high ionic strength media without puromycin; these appear to consist of a mixture of inactive ribosomes and ribosomes containing relatively short nascent chains. A fraction ( N I 5 % ) of the bound ribosomes can only be released from membranes by exposure of R M to ionic conditions which cause extensive unfolding of ribosomal subunits, the nature and significance of these ribosomes is not clear.

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A cell fractionation procedure is described which allows the preparation from rat liver of a rough microsome population containing almost 50% of the membrane-bound ribosomes of the tissue. The fraction is not contaminated with free ribosomes or smooth microsomes, and, by various other criteria, is suitable for studies of ribosome-membrane interaction.

Starting with the portion of partially purified slime mold actomyosin which was voided from Sephadex G-200 at low ionic strength slime mold myosin was further purified by DEAE-cellulose chromatography and salt fractionation with ammonium sulfate. Slime mold myosin had about three times the specific Ca2+-adenosine triphosphatase activity of rabbit myosin assayed under identical conditions. The enzyme was free of nucleic acids and nearly all material present sedimented as a single species with f20iW = 6.4o S in 0.50 m KC1 at pH 7.4. By gel chromatography on Sepharose 4B the enzyme was found to have a high particle asymmetry with a diffusion coefficient similar to rabbit myosin An approximate molecular weight of 4.6 X 105 was obtained. Slime mold myosin formed an actomyosin complex with rabbit actin and was similar to rabbit myosin in some of its enzymatic properties. The main

Extracts of the plasmodium of Physarum polycephalum prepared with various homogenization media contained a calcium-activated adenosine triphosphatase and a rapidly sedimenting actin-like protein. When plasmodial extracts prepared with 0.05 m sodium pyrophosphate-0.01 m Tris-maleate (pH 6.8) were fractionated by precipitation with ammonium sulfate, the adenosine triphosphatase and the rapidly sedimenting protein were isolated as a partially puri-11 has often been suggested that the motile phenomena observed in most biological systems are caused by protoplasmic contractions and that such systems might contain "contractile" proteins similar to the actomyosin isolated from vertebrate skeletal muscle. Indeed numerous workers have claimed the isolation of actomyosin-like proteins from diverse nonmuscular tissues, organisms, and organelles, some of which are not even usually studied in connection with motility (Jahn and Bovee, 1967;Gibbons, 1968). The plasmodium of the myxomycete slime mold, Physarum polycephalum, is a logical choice