Membrane Lipids, Protein Analysis, and Cell Transport

Membrane Lipids and Signaling Molecules

Glycolipids are the least abundant lipids, featuring a backbone made of sphingosine. They contain two tails, usually both saturated, and heads with polar sugar groups. These are always found on the non-cytosolic leaflet, as signaling and recognition occur in the extracellular space. Sterols are the second most abundant lipids, characterized by a rigid ring-structured backbone, one short tail, and a small head group found on both leaflets. There is an asymmetrical distribution of these different types of lipids in the biological bilayer; for instance, Phosphatidylserine (PS), with its negative charge, prefers the cytosolic side due to the reducing environment.

Phosphoinositide Signaling Pathways

Two groups of lipid-modifying enzymes generate PI-derived signaling molecules: PI/PtdIns kinases and phosphatases. PI (Phosphatidylinositol) and PtdIns are phospholipids with a glycerol backbone and a sugar head in the cytosol. They are minor lipid components with tails anchored at the cytosolic leaflets of the plasma membrane (PM) or intracellular membranes. The enzymes that synthesize PI and PtdIns are found in the cytosol. PtdIns is phosphorylated PI at positions 3, 4, or 5. Different kinds of PtdIns depend on the phosphorylation position, such as PI(4)P versus PI(4,5)P2 (phosphorylated at both 4 and 5). PtdIns(4,5) is located on the plasma membrane, while PtdIns(4) is found in the Golgi.

Kinases and phosphatases of these lipids are enzymes that phosphorylate and dephosphorylate PtdIns, changing PI(4) into PI(4,5) or PI(4,5) into PI(4). These conversions are controlled by specific kinases and phosphatases. PtdIns mediate signaling by recruiting specific PtdIns-binding proteins to specific membranes; proteins with domains that can bind to specific PtdIns will be recruited to where that PtdIns is enriched. Phospholipases mediate signaling by being activated by the binding of extracellular ligands to receptors at the cell surface (found mostly on the PM). Once activated, PLC (Phospholipase C) cleaves the phosphodiester bond to create new signaling molecules.

Protein Analysis: SDS-PAGE and Western Blotting

SDS-PAGE is a technique used to separate proteins based on their sizes by applying an electric field in a polyacrylamide gel. Because proteins have different charge-to-mass ratios and shapes, it is impossible to separate them only by mass without using a detergent. SDS (Sodium Dodecyl Sulfate) disrupts hydrophobic and hydrophilic non-covalent interactions, allowing proteins to unfold as extended polypeptide chains. Beta-mercaptoethanol is a reducing agent that reduces disulfide bonds within a polypeptide to ensure proteins have similar shapes. SDS has a hydrophobic hydrocarbon chain and a hydrophilic, negatively charged sulfate group. Only five amino acids have a charge:

• Aspartate
• Glutamate
• Lysine
• Arginine
• Histidine

The SDS charge overwhelms the charge on the protein, giving all proteins a similar charge/mass ratio to separate them solely by size.

Western Blotting and Immunofluorescence

In Western Blotting, the antigen is the molecule recognized by an antibody (most commonly a protein). The primary antibody recognizes the primary antigen and can be either monoclonal or polyclonal. The secondary antibody recognizes the primary antibody (acting as its antigen) and is almost always polyclonal to amplify the signal across multiple epitopes. There are three amplification steps:

1. Original binding of the primary antibody to the target protein.
2. Binding of the secondary antibody to the primary antibody.
3. Amplification of signals via enzyme binding to the secondary antibody.

Western blotting lyses the protein and runs it on a gel to acquire size and signal data, but it does not reveal localization. Immunofluorescence uses a fixed or immobilized antigen. Before adding the primary antibody, the antigen must be fixed, followed by permeabilization of the cell to lock the protein of interest in its physiological location and make the membrane permeable to large molecules like antibodies. Different fluorophores emit and absorb at specific wavelength (WV) ranges, generating different colors to label multiple proteins within a cell.

Membrane Protein Structure and Association

Non-covalent bonds that help proteins fold include electrostatic attraction, hydrogen bonding (hydrophilic interactions), and Van der Waals forces (both hydrophilic and hydrophobic). Integral membrane proteins (transmembrane proteins) have a sequence spanning the hydrophobic part of the entire lipid bilayer. The transmembrane domain adopts either an alpha helix or a beta barrel. An integral membrane protein can have more than one transmembrane domain. The alpha helix is the most abundant because it is stabilized by hydrogen bonds, with the side chains of each amino acid facing outward to contact hydrophobic tails.

Beta-sheets are composed of multiple beta-strands and take up more space than an alpha helix, adopting an alternating up-down conformation. They can be arranged parallel or antiparallel. A beta-barrel consists of multiple beta-sheets forming a hydrophilic pore in the center, with one side of each strand facing the hydrophobic liquid and the other facing the hydrophilic pore. This structure requires fewer amino acid residues to cross the lipid bilayer.

Extracellular and Cytosolic Domains

Disulfide bonds are found between Cysteine (Cys) residues on the luminal side, as the extracellular space is an oxidizing environment. Lipid-anchored membrane proteins are associated with the hydrophobic interior via a covalently attached lipid molecule (the anchor resides in the cytosolic leaflet). These are first synthesized in the ER, and the anchor is added to the C-terminus. Peripheral membrane proteins associate with the membrane via non-covalent interactions with lipids or other membrane proteins, without direct interaction with the hydrophobic core. They may associate via integral membrane proteins (hydrophilic interaction) or lipid molecules like PtdIns.

Mechanisms of Membrane Transport

The transport direction of non-ionic solutes is determined by the chemical gradient, while ionic solutes are determined by the electrochemical gradient (ECG). A transporter mediates either active or passive transport, whereas a channel always mediates passive transport. Energy for active transport commonly comes from light, ATP-driven pumps, or coupled transporters (e.g., Na+/glucose, where Na+ moves down its ECG so glucose can be taken against its ECG). ATP-driven pumps drive transport through phosphorylation or hydrolysis (e.g., the Ca2+ pump uses hydrolysis to transport Calcium against its ECG).

The cytosol has a negative charge, while the extracellular space is positive. Ions in an ion channel move toward their ion equilibrium potential. The cytosol, mitochondria, and ER take up 80% of cell volume. Co-translational transport involves unfolded proteins, while post-translational transport can involve either folded or unfolded proteins. Finally, GAPs (GTPase-activating proteins) inactivate a GTPase by favoring GDP, while GEFs (Guanine nucleotide exchange factors) activate a GTPase by favoring GTP.