Last time... BIOL*1090 Introduction To Molecular and Cellular Biology Fall 2014 Lecture 3 - Sept. 15, 2014 Viruses Biological Membranes Karp 7th ed: Chpt. 4; sections 4-1, 4-3 to 4-7 1 2 VIRUS Non-cellular macromolecular packages that can function and reproduce only within living cells outside of cells, a virus exists as an inanimate particle (= VIRION) Tobacco Mosaic Virus (TMV) VIRION comprised of - small amount of DNA or RNA (encoding a few to hundreds of genes) Fig 1.21 - PROTEIN capsule (= capsid) 4 3 Viruses bind to a cell surface via specific proteins and enter into cell - this defines the cell types the virus can infect and the host range Adenovirus WIDE host range: - rabies can infect cells in dogs, bats, and humans Human immunodeficiency virus (HIV) NARROW host range - human cold and influenza viruses infect epithelial cells of human respiratory system T-even bacteriophage Fig 1.21 5 6
Once inside a cell, the virus hijacks cellular machinery to synthesize nucleic acids and proteins Virus Life Cycle assembles new virus particles Two main types of viral infection: 1) LYTIC: production of virus particles ruptures (and kills) cell (e.g. influenza) 2) NON-lytic or INTEGRATIVE: viral DNA is inserted in host genome DNA = PROVIRUS; viral progeny bud at cell surface; cell can survive, often with impaired function (e.g HIV) 8 7 Function of Biological Membranes cell boundary * The Plasma Membrane define /enclose compartments control movement of material into/out of cell * allow response to external stimuli * Fig 4.1 enable interactions between cells * provide scaffold for biochemical activities ** * plasma membrane only ** including energy transduction Electron micrograph of a muscle cell 9 10 The Fluid-Mosaic model of biological membranes ~ 6 nm thick trilaminar 11 Fluid - individual lipid molecules can move Mosaic - different particles penetrate the lipid layer 12
Biological membranes contain a hydrated lipid bilayer Fluid-mosaic Model (Singer/Nicolson, 1972) - bilayer of amphipathic lipids - proteins: - integral (transmembrane) - peripheral - lipid-anchored components are mobile components can interact Fig 4.3 13 14 all membranes share common properties ~ 6 nm thick (with associated water) - stable - flexible - capable of self assembly different membranes contain different types of lipids and proteins - membranes have different functions, in different cells and within an individual cell 15 An example of differential membrane structure: The inner membrane of mitochondria contains a very high concentration of protein. 16 Electron micrograph of a nerve cell axon (cross section) showing myelin sheath, a modified plasma membrane structure. - Why? The myelin sheath of a neuron contains very low amounts of protein. Myelin sheath consists of layers of plasma membrane, forming insulation around the nerve axon. Fig 4.5 17 18
3 Classes of Membrane Proteins: Different areas of the plasma membrane perform different functions INTEGRAL membrane proteins span the lipid bilayer e.g. epithelial cell LIPID-ANCHORED proteins attach to a lipid in the bilayer PERIPHERAL membrane proteins associate with the surfaces of the lipid bilayer Fig 4.13 19 20 Fig 4.30 Fluidity: an Important Feature of Biological Membranes Membrane fluidity is determined by: nature of lipids in membrane - unsaturated lipids increase fluidity - saturated lipids reduce fluidity temperature - warming increases fluidity Biological Membranes are Asymmetrical two leaflets have distinct lipid composition = liquid crystal in many plasma membranes, the outer leaflet contains glycolipids and glycoproteins (lipids and proteins with carbohydrate attached) - cooling decreases fluidity 21 Transition temperature warm = crystalline gel 22 Membrane Fluidity is Crucial to Cell Function cool BALANCE between ordered (rigid) structure and disordered structure allows: - mechanical support and flexibility Fig 4.23 liquid crystal state crystalline gel state - dynamic interactions between membrane components (e.g. proteins can come together reversibly) - membrane assembly and modification 23 24
Dynamic properties of the plasma membrane Membrane Fluidity is Crucial to Cell Function membrane fluidity must be maintained in response to changes in temperature*, lipid composition of membranes can be changed by: 1) desaturation of lipids 2) exchange of lipid chains Fig 4.8 Ruffles on the plasma membrane of a moving cell Fig 8.45 A leukocyte ingesting a yeast cell (e.g. Listeria monocytogenes changes the lipid content of its plasma membrane while growing at 10 o C) 25 26 Biological Membranes are Dynamic lipids move easily, laterally, within leaflet lipid movement to other leaflet is slow membrane proteins can diffuse within bilayer - movement of proteins is restricted - some proteins do not move - rapid movement is spatially limited - long range diffusion is slow - biochemical modification can dramatically alter protein mobility in the membrane (part of signal transduction) 27