Membranes, Lipids, and Membrane Proteins — Key Concepts

Endoplasmic Reticulum and Golgi: Lipid Biosynthesis, Sorting, and Modification

The endoplasmic reticulum (ER) is a central site for lipid biosynthesis; the smooth ER is the main lipid-generating region, while the rough ER specializes in protein synthesis.
The Golgi apparatus acts as the next station for proteins and lipids that have been synthesized in the ER, directing them to their destinations (cell membrane, other organelles, etc.).
Golgi also modifies lipids and proteins; lipid modification (including glycolipid formation) occurs here, and glycosylation of lipids happens in the Golgi lumen.
Golgi enzymes that modify glycolipids are located on the lumen-facing side, so glycolipids end up on the outer leaflet after trafficking, maintaining membrane topology.
The overall point: lipid biosynthesis begins in the ER, with further processing and sorting in the Golgi, which preserves the orientation/topology of lipids and proteins during transport.

Mitochondria: Lipid Diversity and Membrane Topology

Mitochondria also generate some lipids, but the lecture emphasizes differences between membranes rather than lipid synthesis as a primary focus.
A key feature of mitochondria is their double membrane: an outer membrane (smooth) and an inner membrane (folded). The two membranes differ markedly in lipid composition and protein content.
They illustrate that membranes are not uniform: different organelle membranes, and even the two leaflets within a single membrane, can have distinct lipid and protein compositions.
The inner mitochondrial membrane is highly folded and rich in certain lipids (e.g., cardiolipin) that confer specific curvature and functional properties.

Membrane Structure and Function: The Barrier and Its Components

Primary function of membranes: separate an aqueous interior from an external aqueous environment (and create intracellular compartments in eukaryotes).
The barrier is mainly formed by lipids, with proteins providing selective transport and regulation.
Small nonpolar molecules cross membranes poorly; most transport is mediated by membrane proteins (integral/transmembrane proteins).
Phospholipids are the core membrane components; they assemble by noncovalent interactions with a hydrophobic tail interior and a hydrophilic head.
Phospholipids have a hydrophilic head (often containing a phosphate group) and a hydrophobic tail (fatty acid chains). This amphipathic nature drives bilayer formation with hydrophobic tails inside and hydrophilic heads facing water.
Membranes are composed of two leaflets (bilayer): a cytosolic leaflet in contact with the cytosol and a noncytosolic (lumen/extracellular-facing) leaflet.
Vesicles preserve leaflet orientation during budding and fusion; the cytosolic leaflet on the vesicle remains in contact with the cytosol after fusion with the target membrane.
Because the bilayer would expose hydrophobic interior if open, membranes tend to close into vesicles or tubular structures rather than remain as open sheets.
Lipid bilayers are extremely thin (~5 ext{ nm}), requiring electron microscopy to visualize; light microscopy resolves about 200 ext{ nm}, while fluorescence techniques can reach roughly 20-40 ext{ nm} resolution.

Lipid Diversity and Their Biophysical Consequences

Major lipid types include phospholipids, glycolipids (glycolipids have sugar head groups added in the Golgi), and sterols (cholesterol in animals).
Cholesterol (a sterol) intercalates between phospholipids, modulating membrane rigidity and fluidity and affecting membrane thickness. Its distribution is uneven across membranes: plasma membranes generally have higher cholesterol than many organelle membranes.
The properties of membranes depend on lipid composition:
- Fatty acid chain length (typically 18-22 ext{ carbons}) influences van der Waals interactions and membrane thickness.
- Saturation: saturated tails pack tightly, making membranes less fluid; unsaturated tails with kinks disrupt tight packing, increasing fluidity.
- Cholesterol affects rigidity and thickness in a context-dependent manner shared with other lipids.
Membrane thickness affects protein transmembrane domains: proteins with longer hydrophobic spans are required for thicker membranes. This matching of hydrophobic length is crucial for proper integration.
Cardiolipin: a distinctive, conical phospholipid found predominantly in the inner mitochondrial membrane; typically present in high amounts in the inner leaflet and contributes to membrane curvature and organization in mitochondria.
The inner mitochondrial membrane has unique lipid composition (notably cardiolipin) that helps generate and stabilize curvature in mitochondria.

Lipid Shape, Curvature, and Membrane Organization

Cardiolipin’s conical shape (four fatty acid chains linked to a glycerol) promotes membrane curvature and helps form the highly folded inner mitochondrial membrane.
Lipid shape and composition contribute to the physical properties of membranes, such as rigidity, fluidity, and thickness, which in turn influence protein insertion, orientation, and function.

How Lipids Are Made and Shipped to Membranes

Lipids are hydrophobic; their synthesis occurs in a hydrophilic cytosolic environment, requiring shielding during growth.
Two main carriers deliver lipid building blocks to the ER:
- Coenzyme A (acetyl-CoA) carries acetyl building blocks for fatty acid synthesis; fatty acids grow two carbons at a time, leading to even-numbered carbon chains (no 17- or 19-carbon fatty acids in cells).
- Fatty acid binding proteins (FABPs) shuttle fatty acids in the cytosol with a hydrophobic pocket for transport through the aqueous cytosol.
Final assembly of phospholipids occurs at the ER on the cytosolic leaflet; growth of the cytosolic leaflet expands the membrane.
To expand the noncytosolic leaflet, scramblases move lipids non-selectively between leaflets, equalizing membrane surface areas.
To create lipid asymmetry, flipases (lipid-specific scramblases) move a subset of lipids from the cytosolic leaflet to the noncytosolic leaflet, generating leaflet asymmetry.
Lipids are not created de novo in the plasma membrane; they are added to existing membranes and then redistributed as needed.

Lipid Transport and Membrane Proteins: Integral, Peripheral, and Lipid-Anchored

Membrane proteins come in several flavors:
- Integral membrane proteins: span the membrane with transmembrane domains; can be single-pass or multi-pass (up to 16 domains in some proteins).
- Transmembrane domains are typically hydrophobic to interact with the lipid bilayer; transmembrane regions are often alpha helices (common) or beta barrels (common in outer membranes).
- Peripheral membrane proteins: do not insert into the bilayer; they associate with membrane lipids or with other membrane proteins and can bind to cytosolic or extracellular domains.
- Lipid-anchored proteins: covalently bound to lipid molecules; GPI anchors attach proteins to the noncytosolic leaflet via a glycolipid, whereas other lipid anchors can insert into the cytosolic leaflet.
Topology of membrane proteins is essential: orientation (which terminus faces cytosol vs. extracellular space) is preserved during insertion and trafficking; improper orientation disrupts function.
Transmembrane helices are typically about 20-25 ext{ amino acids} long to span a membrane; thicker membranes may require longer spans. The amino-terminal and carboxy-terminal orientations determine whether the N- or C-terminus faces the cytosol or extracellular space, depending on the protein’s topology.
Beta-barrel proteins (beta sheets forming a pore) are common in outer membranes (e.g., outer mitochondrial membrane) and have alternating hydrophobic/hydrophilic residues to create a hydrophilic pore inside and hydrophobic exterior.
A prominent class of multi-pass receptors is the seven-transmembrane-domain proteins (e.g., rhodopsins). These domains typically insert perpendicularly to the membrane; some proteins have oblique orientations requiring longer segments to traverse the hydrophobic core.

Transport Across Membranes: Passive, Active, and Coupled Mechanisms

Passive transport: molecules move down their concentration gradient via channels or transporters.
- Channels: form pores; allow passage of specific molecules based on size and charge; gated channels open or close in response to signals.
- Transporters: bind the molecule and undergo conformational changes to release it on the other side; still move with the concentration gradient.
Active transport: moves molecules against their gradient, requiring energy input.
- ATP-driven pumps: hydrolyze ATP to move molecules against the gradient.
- Gradient-driven pumps (coupled transport): use the energy from moving one molecule down its gradient to move another molecule up its gradient.
- Antiporters: move two different molecules in opposite directions (e.g., one down its gradient, the other up).
- Symporters: move two molecules in the same direction (both along their gradients or one against, one with, depending on coupling).
A note on compatibility: many membrane proteins perform multiple roles, such as receptors that also function as channels or enzymes that also act as transporters; functional categories are not mutually exclusive.

Mobility and Organization in the Membrane

Lipids are highly mobile within each leaflet (lateral diffusion); proteins are also mobile but their movement can be constrained by interactions with the cytoskeleton, extracellular matrix, or neighboring cells.
Techniques to study mobility include fluorescence recovery after photobleaching (FRAP), demonstrating lateral diffusion of lipids and proteins in membranes.
Two-dimensional fluid model: membranes are dynamic, but diffusion can be restricted by protein–lipid interactions, cytoskeletal attachments, and cell–cell junctions.
Tight junctions create a barrier to lateral diffusion between apical and basolateral surfaces in epithelial cells, maintaining compartmentalization (e.g., glucose transport in intestinal epithelium requires separation of transporter proteins on different sides).
Anchoring and interactions with cytoskeleton or extracellular components further restrict mobility and maintain membrane domains.

Practical Implications: Membrane Architecture and Function

Membranes are specialized barriers that create organelle compartments and regulate molecular traffic across membranes.
The lipid composition and leaflet asymmetry tailor membrane properties (rigidity, thickness, and fluidity) to organelle-specific needs.
Proper lipid and protein topology is essential for function; mislocalization or incorrect orientation can impair signaling, transport, and metabolism.
Vesicle trafficking relies on maintaining leaflet orientation, proper lipid modification in the Golgi, and selective lipid redistribution to preserve organelle identity and function.

Quick Summary of Key Quantities and Concepts

Membrane thickness: approximately 5 ext{ nm}.
Light microscopy resolution: approximately 200 ext{ nm}; fluorescence methods can reach about 20-40 ext{ nm}.
Fatty acid chain length in membranes: typically 18-22 ext{ carbons}.
Phospholipid topology and leaflets: two leaflets per membrane; cytosolic and noncytosolic orientations are preserved during trafficking.
Alpha-helix transmembrane domains: typically 20-25 ext{ amino acids}.
Seven-transmembrane-domain proteins: a major class exemplified by rhodopsins; domains may be perpendicular or obliquely oriented, affecting the length requirement of the transmembrane span.
Beta-barrel transmembrane proteins: comprised of ~16 beta strands forming a pore; common in outer mitochondrial membranes.
Cardiolipin: a conical phospholipid with four fatty acid chains, enriched in the inner mitochondrial membrane, contributing to curvature.
Lipid asymmetry: scramblases move lipids non-selectively between leaflets; flipases move specific lipids to establish asymmetry.
Cholesterol: sterol intercalating into membranes, modulating fluidity and thickness; uneven distribution across membranes.
Golgi: critical for glycosylation of lipids (e.g., glycolipids) and for enforcing correct lipid topology and leaflet distribution.
Membrane organization in tissues: tight junctions and cytoskeleton interactions create and maintain membrane domains and restrict diffusion.

Key Concepts and Connections

Lipids provide the barrier and mechanical properties of membranes; proteins provide selective transport and signaling capabilities.
The ER-Golgi-mitochondria axis illustrates how membranes grow and mature through non-de novo synthesis, requiring phospholipid transfer and leaflet expansion with regulatory enzymes (scramblases and flipases) to maintain asymmetry.
Membrane curvature and topology are intimately tied to lipid shape (e.g., cardiolipin’s conical shape) and to the distribution of lipids across leaflets.
The two types of transport across membranes (passive vs active) are mediated by distinct classes of proteins with specialized structures (channels, transporters, pumps) and are essential for cellular homeostasis.
The topology of membrane proteins is preserved through trafficking, which ensures correct interaction with extracellular partners and intracellular targets; misorientation could impair signaling, transport, or enzymatic activity.
Experimental evidence (e.g., FRAP and cell fusion studies) demonstrates lateral mobility of lipids and proteins, forming the basis for understanding membrane microdomains and functional organization.