BMB 251 1st Edition Lecture 30 Outline of Last Lecture I. Lipid bilayer ProteinsII. Transmembrane ProteinsIII. Cytosolic ProteinsIV. Noncytosolic ProteinsV. Types of Transmembrane ProteinsVI. Sugar-Coated Membranes Outline of Current Lecture VII. Clicker QuestionsVIII. DetergentsIX. FRAPX. Tight JunctionXI. Transport ProteinsXII. Channel ProteinsXIII. Passive TransportXIV. Active TransportCurrent Lecture- Clicker Question 1: The transmembrane domain of a single-pass transmembrane protein usually has which of the following characteristicso It is an alpha helix Beta sheets normally create barrels, which are mulitpass transmembrane proteins It cannot be enriched in polar residues because the bilayer is hydrophobic, and so there must be a greater amount of nonpolar residues than polar ones to interact- Clicker Question 2: Which would be faster, a transporter protein or a channel protein?o A channel protein Transporter proteins have to change their conformational orientation in order toallow molecules through, whereas channels do not have to change conformationin order to allow molecules through the membrane Channels allow molecules to diffuse through the membrane (much quicker than binding and changing conformation) and allows many molecules through at once- Only agents that disrupt hydrophobic associations and destroy bilayer can solubilize transmembrane proteinsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Detergents: small amphiphilic molecules which are much more soluble in water then lipids areo Polar ends can be charged (Ex. Ionic SDS) or uncharged (Ex. Nonionic octylglucoside)o Monomeric in solution, form aggregated micelles at critical micelle concentration (CMC)o Hydrophobic ends of detergent bind with hydrophobic regions of proteins and displace lipids around themo Strong detergent unfold even the most hydrophobic proteins  can be studied (usually through SDS polyacrylamide-gel electrophoresis)- Membrane proteins function as part of multicomponent complexeso Multiple polypeptides associate in membrane to form complex protein machine- Membrane proteins do not tumble (flip-flop) across bilayer, but rather rotate about an axis perpendicular to the bilayer (rotational diffusion) or laterally move within the membrane (lateral diffusion)- Fluorescence Recovery After Photobleaching (FRAP): technique used to measure lateral diffusionrates of membrane proteinso Mark membrane protein of interest with specific fluorescent group (can use fluorescent ligand or recombinant DNA fused with GFP)o Fluorescent group is bleached in a small area of membrane via laser beamo Time taken for adjacent membrane proteins carrying unbleached ligand of GFP to diffuseinto the bleached area is measured o Drawback: cannot follow individual protein molecules because FRAP only monitors the movement of large populations of molecules- Single-particle tracking: solves above problem by labeling individual membrane molecules with antibodies coupled to fluorescent dyes or tiny gold particles and tracking their movement- Cells can confine proteins and lipids to specific domains within a membrane o In epithelial cells (lining the gut and kidney tubules), some plasma membrane enzymes and transport proteins are confined to apical surface of cells and others are confined to basal and lateral surfaces Can prevent diffusion of certain molecules across membrane Tight junction: barriers set up by a specific type of intercellular junction that maintain the separation of both protein and lipid molecules **Proteins forming this junction cannot diffuse laterally in interacting membraneso Sperm cell: several structurally and functionally distinct parts covered by a continuous plasma membrane  contains three distinct domains Some membrane molecules are able to diffuse freely within the confines of theirown domaino Four common ways of immobilizing specific membrane proteins through protein-proteininteractions Proteins can self-assemble into large aggregates  Proteins can be tethered by interactions with assemblies of macromolecules outside the cell Proteins can be tethered by interactions with assemblies of macromolecules inside the cell Proteins can interact with proteins on the surface of another cell- Lipid bilayer is hydrophobic and therefore prevents the passage of many polar molecules  need special proteins for this- The smaller the molecule and the more hydrophobic it is, the more readily it will diffuse across the lipid bilayer (O2 and CO2)- Charged ions such as Na+ and K+ almost never cross the bilayer on their own- Membrane transport proteins (aka “carriers” and “permeases”): transfer polar molecules (such as ions, sugars, amino acids, nucleotides and many metabolites) across the membraneo Very specific to only allow transfer of the specific molecules they encode for across the bilayero Multipass transmembrane proteinso Bind a specific solute to be transported and undergo a series of conformational changes to transfer solute across the membrane- Channels: interact more weakly with solute to pass straight through a much faster rates than transporters - Passive transport or facilitated diffusion: uses concentration gradient (difference in concentration on two sides of membrane) to drive process in a passive or “downhill” direction- When solute carries a net charge, both concentration gradient and electrical potential difference(membrane potential) influence its transport  combine to form electrochemical gradient (net driving force for each charged solute)o Favors entry of positively charged ions into cell, but opposes entry of negatively charged ions- Active transport: process of pumping certain solutes across membrane against electrochemical gradients (“uphill”), which is mediated by transporters  tightly coupled to a source of ATP or ion gradients- **Transmembrane movement of small molecules mediated by transporters can be active OR passive, while channels are always