BIO4212 Membrane Structure Part 2

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Questions and Answers

What is a characteristic of single-pass transmembrane proteins?

They contain multiple transmembrane segments.

They contain only polar amino acids.

They have one α-helix traversing the lipid bilayer. (correct)

They must be energetically favorable to transfer molecules.

What distinguishes multipass transmembrane proteins from single-pass proteins?

They consist of several α-helices that span the lipid bilayer. (correct)

They have a soluble form when not embedded.

They exist only as dimers.

They require energy for molecule transfer.

Which structural feature is crucial for the passage of water molecules through aquaporin channels?

Presence of 5 α-helices.

Tetramer formation with four monomers. (correct)

Configuration of a single α-helix.

A large β barrel structure.

What type of interactions are important for the dimerization of Glycophorin A?

Electrostatic interactions. Signup and view all the answers

Which of the following best describes the properties of β barrels in multipass transmembrane proteins?

They are rigid and found in the outer membranes of certain organelles. Signup and view all the answers

What is a characteristic of peripheral membrane proteins?

They can be easily solubilized by extreme pH. Signup and view all the answers

How do lipid anchors affect membrane proteins?

They control the localization of some signaling proteins. Signup and view all the answers

What type of layer does the glycoproteins form on the extracellular surface of the cell membrane?

Glycocalyx Signup and view all the answers

Which property distinguishes transmembrane α-helices from peripheral proteins?

Transmembrane α-helices span the lipid bilayer. Signup and view all the answers

What role do polar amino acids play in the structure of membrane proteins?

They line the channels of β barrels to facilitate transport. Signup and view all the answers

Which term refers to long polysaccharide chains that are linked to proteins?

Proteoglycans Signup and view all the answers

What is the main function of the glycoproteins and proteoglycans within the glycocalyx?

Cell recognition and adhesion Signup and view all the answers

How are Rab proteins categorized based on their attachment mechanisms?

Myristoylated Signup and view all the answers

What primarily characterizes the Src family of proteins?

Tyrosine kinase activity Signup and view all the answers

What role does the glycocalyx play in cellular interactions?

Prevents unwanted cell interactions Signup and view all the answers

Which of the following is NOT a function of the glycocalyx?

Regulating intracellular signaling Signup and view all the answers

What type of molecules are generally used by the Ras family for membrane attachment?

Palmitic or prenyl acids Signup and view all the answers

What role do detergents play in the solubilization of membrane proteins?

They compete with lipid interactions to create complexes. Signup and view all the answers

Which property distinguishes mild nonionic detergents from harsher detergents?

They solubilize proteins without unfolding them. Signup and view all the answers

What is one application of purified membrane proteins after solubilization?

To study their roles in biological systems. Signup and view all the answers

What structural characteristic do nanodiscs provide for studying membrane proteins?

Uniformly sized patches of membrane surrounded by a protein belt. Signup and view all the answers

Bacteriorhodopsin is notable for which of the following features?

It actively pumps H+ ions and produces ATP. Signup and view all the answers

Why is single-particle electron microscopy a valuable technique for studying membrane proteins?

It enables observation of proteins in their native bilayer environment. Signup and view all the answers

What happens to protein mobility within the cell membrane?

Lateral diffusion is limited to specific domains. Signup and view all the answers

How are detergent micelles formed during the solubilization of membrane proteins?

When detergent molecules reach critical micelle concentration. Signup and view all the answers

What is primarily restricted to the apical surface of epithelial cells?

Transport proteins and enzymes Signup and view all the answers

How can membrane proteins restrict their lateral diffusion?

Through self-assembly into aggregates Signup and view all the answers

What role does spectrin play in the erythrocyte membrane?

It provides mechanical strength and elasticity Signup and view all the answers

Which proteins are involved in the attachment to the basal plasma membrane of epithelial cells?

Adhesion proteins Signup and view all the answers

What is a function of membrane-bending proteins?

To deform lipid bilayers and increase surface area Signup and view all the answers

Which mechanism is NOT involved in restricting lateral diffusion of membrane proteins?

Protein clustering beyond the membrane Signup and view all the answers

What distinguishes intestinal epithelial cells from other cells?

Differences in proteins of apical, lateral, and basal membranes Signup and view all the answers

What feature defines transmembrane proteins?

They can be either single-pass or multi-pass proteins Signup and view all the answers

What is the primary function of lipid rafts in cellular membranes?

Facilitation of signal transduction and protein interactions Signup and view all the answers

Which process do lipid rafts NOT participate in?

Exocytosis Signup and view all the answers

What characterizes the lipid composition of the outer monolayer in cell membranes?

Contains phospholipids with choline heads Signup and view all the answers

How do lipid droplets influence membrane synthesis?

By serving as building blocks of neutral lipids Signup and view all the answers

What is the role of phosphatidylserine translocation during apoptosis?

To signal for macrophage phagocytosis Signup and view all the answers

Which type of receptors are typically found in lipid rafts?

G-protein coupled receptors Signup and view all the answers

What is NOT a characteristic of lipid droplets?

Serves exclusively for lipid metabolism Signup and view all the answers

The difference in lipid compositions between the inner and outer monolayers results in what phenomenon?

Membrane asymmetry Signup and view all the answers

What processes are associated with lipid rafts during endocytosis?

Caveola formation and vesicle formation Signup and view all the answers

Which enzyme is involved in lipid metabolism and associated with lipid droplets?

Phospholipase C Signup and view all the answers

What is primarily found in the inner monolayer of cell membranes?

Phospholipids with terminal primary amino groups Signup and view all the answers

What defines the mechanism of action in b cell receptors when interacting with an antigen?

Dimerization in confined areas of the membrane Signup and view all the answers

What constitutes the fundamental role of a scramblase in the membrane?

Translocating lipids between monolayers non-specifically Signup and view all the answers

Study Notes

Membrane Structure and Function

Lipid Rafts: Domains in membranes where lipids and proteins are concentrated, facilitating protein-protein, protein-lipid, and lipid-lipid interactions.
Signal Transduction: B cell receptors dimerize in lipid rafts upon antigen interaction, initiating signaling for antibody production.
Endocytosis: Lipid rafts play a role in endocytosis through caveola and transport vesicle formation, containing G-protein coupled receptors aiding in neurotransmitter transport.

Lipid Droplets

Lipid droplets store excess neutral lipids, serving as building blocks for membrane synthesis or energy.
Encased by a phospholipid monolayer with associated enzymes critical for lipid metabolism.
Created by budding from the endoplasmic reticulum (ER), found in adipocytes and fatty-acid-rich cells.

Monolayer Composition Differences

Outer Monolayer: Comprises phosphatidylcholine and sphingomyelin with choline heads.
Inner Monolayer: Contains phosphatidylethanolamine and phosphatidylserine, generally negatively charged.
Proteins like Protein Kinase C (PKC) interact with lipids on the inner monolayer.

Phosphatidylserine Translocation

Apoptotic cells translocate phosphatidylserine from the inner to the outer layer, signaling macrophages for phagocytosis.

Membrane Protein Types

Single-pass Transmembrane Proteins: Contain one α-helix that spans the membrane.
Multipass Transmembrane Proteins: Multiple α-helices traverse the lipid bilayer, crucial for channel or transporter functions.

Aquaporin Channel

Functions as a water channel, formed by a tetramer of four monomers, each with six α-helices, with NPA motif for substrate specificity.

β Barrels in Proteins

Multipass transmembrane proteins form β-barrels, commonly found in bacterial outer membranes, mitochondria, and chloroplasts.
Channel functions facilitated by polar side chains lining the protein pores.

Peripheral Membrane Proteins

Located outside the lipid bilayer, attached via amphipathic α-helices or covalent lipid modifications.
Solubilized easily by salt solutions or extremes in pH.

Lipid Anchors and Protein Localization

Lipid anchors (e.g., myristoylation) dictate membrane localization of signaling proteins.
Src family tyrosine kinases and Ras family GTPases are examples of proteins linked to membranes via lipid modifications.

Sidedness of Membrane Proteins

Membrane protein orientation assessed through techniques like electrophoresis, demonstrating distinct upper and lower domains.

Glycoproteins and Glycocalyx

Glycoproteins feature oligosaccharides or polysaccharide chains, contributing to the glycocalyx that serves protective and adhesive functions; found on the extracellular surface.

Detergent Use in Membrane Protein Studies

Detergents solubilize membrane proteins while maintaining functionality.
Mild nonionic detergents can isolate proteins for functional studies, allowing reconstitution into nanodiscs.

Bacteriorhodopsin Functionality

Acts as an H+ pump identified by electron crystallography, composed of seven α-helices and essential for ATP production.

Protein Domain Restrictions

Protein movement is restricted to specific membrane domains; for instance, in epithelial cells, proteins are localized to apical or basal surfaces.

Mechanisms of Protein Diffusion Restriction

Self-assembly into aggregates, tethered interactions with macromolecules, or interactions with neighboring cell proteins.

Cortical Cytoskeleton Structural Role

Spectrin forms a membrane skeleton, maintaining erythrocyte shape and elasticity through interactions with band 3 and other cytoplasmic proteins.

Membrane-Bending Proteins

Proteins can deform lipid bilayers through hydrophobic domain insertion, lipid anchor attachment, and clustering of membrane lipids.

Summary

Membrane function primarily ascribed to diverse transmembrane proteins.
Proteins exhibit complex arrangements and roles across the bilipid layers, contributing to cellular functions and signaling pathways.

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Description

This quiz focuses on the complex structure of membrane lipids and proteins as discussed in the second part of the membrane structure course. Topics include lipid rafts, the interactions among various membrane components, and their roles in cellular signaling and transport. Prepare to explore the fascinating mechanisms that govern membrane dynamics.