Membrane structure

Chemistry 256

Membranes

• Principal function is to keep unwanted materials outside of cell (or other area) and, secondarily, to allow needed materials in.

• London interactions of glycerophospholipid tails creates a nonpolar environment in the middle of membrane, impermeable by polar materials

Micelle – single glycerophospholipid layer (lipid is effective modeled as a triangle) – creates

nonpolar environment where tails meet

On the other hand, a solution of sphingolipids or glycerophosholipids will spontaneously form liposomes – closed, self-sealing solvent-filled vesicles, stable and purifiable. The internal environment of a liposome may be quite different than the external environment.

Membrane – glycerophospholipid bilayer with proteins (lipid molecules are modeled as

rectangles, allowing antiparallel layers of lipids – measured 3.3 nm thick by Hugo Fricke

(Cleveland Clinic) in 1925)

Properties of membranes – lipids can easily move laterally within same layer,

but not “flip” sides (switch layers)

Enyzmes mediate flipping

Distribution of glycerophospholipids on different layers of bilayers is not similar – a function of what properties are needed

by each side

Mechanisms for moving glycerophospho-lipids from one side to the other

• flipases move lipids along their concentration gradient (facilitated diffusion)

• phospholipid translocaes move lipids against their concentration gradient (active transport)

Melting (transition) point of a membrane – temperature above which lipids lose

quasi-crystalline state

Cholesterol increases melting point of membrane by providing a rigid planar surface against which lipids adhere

Other membrane components – proteins

Integral (intrinsic) proteins are tightly associated with the membrane. For instance, trans-membrane proteins go through the membrane. Note highly nonpolar residues are found exclusively within the membrane. Note also the glycosylation of residues outside the membrane.

Transmembrane proteins are often used to allow certain molecules through the

membrane – thus they need to define a hole

-barrelaligned -helices

Key points about membrane-spanning transport proteins

• With aligned -helix structure, the transported molecule or ion does not go through an individual helix; instead, goes through the space defined in the middle of the aligned helices.

-barrels are found in porins, which use a passive transport, diffusion-aided mechanism; -strands are antiparallel

Integral proteins are sometimes attached to a membrane using a lipid group

• Prenylated proteins (using isoprenoid groups - attached through a serine residue below)

• Fatty acylated proteins (through glycine)

• Glycosylphosphatidylinositol-linked (GPI-linked) proteins (through C-terminus)

Peripheral (extrinsic) proteins – not held tightly to membrane surface

Cytochrome c is loosely associated with the outer surface of the inner mitochondrial membrane; it is a cation and thus can interact with negatively charged phospholipids. It can be removed without damaging the membrane.

Membrane structure• SJ Singer and GL Nicolson (UC San Diego – 1972), The

fluid mosaic model of the structure of cell membranes, Science 175

• Basic idea: membranes are a lipid “sea” in which integral proteins “float”

From the abstract: “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.”

Evidence for the model

• Earlier model (Davson-Danielli – 1935) used electron micrography of membrane (dark-light-dark pattern) as evidence for a rigid three-layer (lipid-protein-lipid) model

• Experiments with fusing cells with fluorophore-tagged integral proteins showed these proteins had excellent mobility – membranes must be fluid

• Experiments extracting transmembrane proteins showed the proteins contained hydrophobic sequences – no need for a separate layer for proteins

Further structural considerationsErythrocytes retain their “dimple” even as lipids and proteins “flow” – explanation: a proteinaceous sub-membranous skeleton (actin and spectrin microfilaments (Ralston, 1976) anchored by ankyrin (Bennett, 1979) to the membrane). Spectrin has 2 subunits, each made of triple helices.

Actin is a 42-kD highly-conserved globular protein which folds and links with other actin to form microfilaments.

Even further structural considerations

• Lipid rafts, made of associations of glycosphingolipids and cholesterol represent a more crystalline area of the bilayer and are implicated in the construction of signaling complexes on the membrane

Final structural consideration• Proteins, unlike lipids, cannot

translocate (change orientation) in a membrane, must be manufactured correctly in the first place; for instance, a particular protein’s N-terminus must always be facing out. How?

• Recall membrane assembly begins in the rough endoplasmic reticulum (RER); this is where the protein is synthesized and placed in the membrane

Vesicles are generated with the help of a protein (clathrin) framework

Clathrins are built from proteins called triskelions, which interlink to give a three-dimensional shape. Interestingly, lipids themselves, when compressed, form triskelion-like solid structures.

Figure 9-40

The vesicles themselves are coated with proteins such as COP I to recognize other membrane surfaces and to orient the vesicle membrane correctly (inside/outside) with the target membrane.

The secretory pathway: Interaction of the translocon protein and the signal

recognition particle (SRP)

Once the protein is inserted...

• The part of the RER pinches off, detaches, and moves to the Golgi apparatus, where the ultimate destination of the package is decided

• The Golgi bodies contain glycosylation enzymes, which is why only the part of the protein (for vesicles destined to be part of the plasma membrane) that is topologically equivalent to the “outside” are glycosylated

• Carbohydrates on glycosylated proteins play role in directing vesicles to proper destination

And once at the plasma membrane...• The protein part expressed

in the lumen of the vesicle plays a major role in vesicle-membrane fusion because it is these proteins that allow each membrane to recognize the other

• Soluble N-ethylmaleimide sensitive fusion Attachment protein REceptor (SNARE) is critical for this recognition; botulism and tetanus toxins are proteins that interfere with neurotransmitter-containing vesicles