Lecture series D5

“Membrane structure and membrane transport”

notes based on Alberts et al 4^th ed. (2002) Chapters 10 and 11

prepared by T. J. Newman, November 6-November 8, 2005

this document not for public use – all images copyright Garland Science Publishing 2002

INTRODUCTION

· The cell membrane is composed of the lipid bilayer and numerous membrane proteins

· Eucaryotic cells contain large amounts of membrane in the cytoplasm, in structures such as the endoplasmic recticulum and mitochondria

· It is estimated that 30% of protein types in eukaryotes are membrane proteins

· These proteins can maintain ion gradients to help create ATP, drive the movement of selected solutes, or transmit electrical signals

· Receptor proteins also transmit information to the cell concerning the nature of the extracellular environment

LIPID BILAYER

· The lipid bilayer is the universal basis for cell membrane structure

· 50% of the mass of the membrane is in the form of lipid (fatty) molecules

· The membrane of a small animal cell contains about 10⁹ lipid molecules

· Most lipids are phospholipids

o these have a polar (hydrophilic) head group and two hydrocarbon (hydrophobic) tails

o one tail is saturated, the other contains double bonds which introduces kinks in the tail

o this influences the rigidity of the membrane and affects the diffusivity of proteins within the membrane

· The amphipathic nature of phospholipids leads them to spontaneously form structures in water

o minimizing the disruption of hydrogen bonding

· “Wedge-shaped” lipids tend to form spherical micelle structures

· “Cylindrical: phospholipids form bilayer-sheets

o these sheets can spontaneously fold up into spherical vesicles to minimize costly edge-water interactions

· The lipid bilayer acts as a two-dimensional fluid

o lipid molecules readily exchange with their neighbors in the same monolayer

§ diffusion coefficient for this motion has been estimated at 10^-8 cm²/s

§ this is equivalent to a lipid diffusing the length of a 2μm bacterial cell in about 1 second

o in contrast lipid molecules rarely cross over from one lipid monolayer to the other (a process known as “flip-flop”)

o interestingly, lipid molecules are only constructed in the “inner” (cytosolic) monolayer

§ appropriate numbers of lipids are transported to the outer monolayer by special proteins known as phospholipid translocators

· The fluidity of the bilayer membrane depends sensitively on temperature and the composition of the membrane

o the transition from a liquid state to a rigid “gel” state is sharp (as in a phase transition)

o membranes are more fluid if hydrocarbon tails are shorter, or contain more double bonds

· Microorganisms, whose temperature fluctuates with that of the environment, can adjust the composition of the lipid hydrocarbons so as to preserve a relatively constant fluidity of the membrane

· Eucaryotic cell membranes contain a large amount of cholesterol

o cholesterol molecules align themselves in the membrane with their polar group close to the polar groups of the lipids

§ this enhances the rigidity of the membrane and makes it less permeable to small solutes

· There are several types of phospholipids found in eucaryotic cell membranes

o predominant types are: phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin

o the relative amounts of these different lipids in the membrane has a strong effect on the function of membrane proteins

§ e.g. phosphatidylserine is negatively charged which affects binding of certain enzymes

· Sphingolipids tend to have longer saturated hydrocarbon tails

o as such these lipids can form small rigid domains of lipids, known as “lipid rafts”

o these rafts are thicker than typical membrane and thus attract certain types of transmembrane protein which need a thicker membrane within which to embed themselves

o thus rafts can spatially localize and organize certain proteins

· There is great asymmetry as to the types of phospholipids found in the inner and outer lipid monolayer

o this allows proteins to preferentially bind either to the cytosolic monolayer or the noncytosolic monolayer

· This asymmetry is also used by cells to distinguish live and dying cells

o during apoptosis, phosphatidylserines, which are usually in the inner monolayer, are translocated to the outer monolayer, and this signals to other cells that this cell is dying and should be digested by macrophages

· Another type of lipid are the glycolipids (lipids containing sugar)

o these are found exclusively in the outer (noncytosolic) monolayer

MEMBRANE PROTEINS

· Typical membranes contain, by mass, 50% lipids and 50% proteins

o which corresponds, on average, to 1 protein per 50 lipid molecules

· While lipids provide mainly structural integrity, the membrane proteins provide biochemical function

· Transmembrane proteins extend through the bilayer, with hydrophobic regions interacting with the lipids, and polar groups interacting with the water on either side

· Alternatively, membrane proteins may only embed themselves into the cytosolic monolayer, or else attach to either monolayer through covalent bonds to fatty acids

· Still other proteins bind to transmembrane proteins

· Naturally, the type of protein-lipid interaction is determined by the necessary function of the protein

o e.g. cell-surface receptors need to be triggered by an extracellular signal, and then transmit this information to the intracellular environment – thus, such proteins are transmembrane proteins

o intracellular signaling proteins may only be embedded in the cytosolic monolayer

· Peptide bonds favor hydrogen bonding, and will bind to each other within the water-free membrane

· Transmembrane proteins typically use a-helices to wind their way (sometimes multiply) through the bilayer membrane

o e.g. bacteriorhodopsin – a transmembrane protein found in the “purple membrane” of the archaean Halobacterium salinarum

o this protein traverses the bilayer with 7 helices

· This protein pumps protons across the membrane when stimulated by photons

o this gradient is used to power the production of ATP

o note, the molecule retinal is also used in rhodopsin proteins in the photoreceptor cells of the vertebrate eye

o however, the sequences of bacteriorhodopsibn and mammalian photo-signaling receptors show no similarity, indicating they belong to two different branches of the evolutionary tree

· Larger transmembrane “pores” are formed from “b barrels”

o the number of b strands can vary from as few as 8 to as many as 22 (see below)

· These proteins use b sheets to efficiently create a low-energy structure within the hydrophobic bilayer membrane

o b barrels tend to be more rigid, and thus less difficult to crystallize – hence more is known about their structure

o e.g. porins have been well-studied – these form large membrane pores

§ they have a particular internal structure of polypeptide chain which creates narrow regions, thus allowing extreme selectivity as to which molecules can diffuse through the pore

§ e.g. maltoporin selectively allows the transport of maltose across the membrane of bacteria

o another example is the FepA protein

§ this has a large b barrel (22 strands) with a globular domain inside

§