RNA interference (RNAi) is the process of sequence-specific, post-transcriptional gene silencing in animals and plants, initiated by double-stranded RNA (dsRNA) that is homologous in sequence to the silenced gene. The mediators of sequence-specific messenger RNA degradation are 21- and 22-nucleotide small interfering RNAs (siRNAs) generated by ribonuclease III cleavage from longer dsRNAs. Here we show that 21-nucleotide siRNA duplexes specifically suppress expression of endogenous and heterologous genes in different mammalian cell lines, including human embryonic kidney (293) and HeLa cells. Therefore, 21-nucleotide siRNA duplexes provide a new tool for studying gene function in mammalian cells and may eventually be used as gene-specific therapeutics.

Duplexes of 21-nucleotide RNAs mediate RNA interference in cultured mammalian cells

A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation.

The fluid mosaic model of the structure of cell membranes.

THE FLUID MOSAIC MODEL OF THE STRUCTURE OF CELL MEMBRANES Reprinted with permission from Science, Copyright AAA, 18 February 1972, Volume 175, pp. 720–731.

Commentary. Lysosomotropic agents.

The study of endocytosis has traditionally focused on the contents of endocytic vacuoles, i.e., extracellular fluid, dissolved solutes, and macromolecules or particles which specifically or nonspecifically bind to the plasma membrane (PM). Substances that are endocytosed include important nutrients, toxins, effector molecules (growth factors, hormones, antibodies), enzymes, and pathogens. This article centers on the properties and dynamics of the endocytic vacuole membrane. In many instances we will stress observations on cultured mouse macrophages with which we are most familier. We will emphasize four points: (a) Movement of vesicles is rapid such that endocytosed membrane and contents move from one cellular compartment to another in seconds to minutes. (b) Vesicular movement requires the interiorization and flow of large amounts of PM) (c) In many instances, internalized PM must recycle or return intact to the cell surface. (d) During recycling, contents and membrane components can be sorted from one another; e.g., endocytosed contents can accumulate within the cell while the container (membrane) can move into and out of the cell after one or more fusion events with other endocytic vacuoles, lysosomes (Ly), or Golgi apparatus. While it has been difficult to obtain direct evidence, the literature is replete with examples in which rapid membrane flow and recycling readily explains the data. Two of the more striking examples derive from studies of pinocytosis in cultured ceils. Fibroblasts, for instance, interiorize the equivalent of 50% of their surface area and 5-10% of their cell volume during each hour of pinocytic activity. Yet, the overall dimensions of the cells and the vacuolar system remain constant throughout hours, even days, of endocytic activity. Since it is unlikely that internalized PM is rapidly degraded, it was proposed that

/pdf/endocytosis-and-the-recycling-of-plasma-membrane-ivaf90oe9b.pdf

Endocytosis and the recycling of plasma membrane.

Antibody reacting with lymphocyte surface immunoglobulin molecules induces these to gather over one pole of the cell. This suggests a possible mechanism for lymphocyte triggering by antigen and raises questions about cell membrane structure.

Redistribution and Pinocytosis of Lymphocyte Surface Immunoglobulin Molecules Induced by Anti-Immunoglobulin Antibody

https://jcp.bmj.com/content/jclinpath/27/Suppl_7/31.full.pdf

Stefanello de Petris

Papers

Redistribution and Pinocytosis of Lymphocyte Surface Immunoglobulin Molecules Induced by Anti-Immunoglobulin Antibody

Ligand-induced redistribution of membrane macromolecules: some possible implications for cancer