Midterm No. 2 Study Questions Flashcards
Although the lipid bilayers are fluid, the plasma membrane is asymmetrical in the composition of the monolayers (leaflets). Is this a paradox?
No. Lipids can move laterally within the leaflets due to the lack of covalent bonds between the two leaflets (transverse movements are highly restricted though)
Leaflet composition doesn’t affect fluidity
Do bacteria have cholesterol?
No
Which plasma membrane leaflet has glycoproteins and glycolipids?
Outer/exterior
Which plasma membrane leaflet (normally) has phosphatidylserine?
Interior
Which plasma membrane leaflet (under extreme cell stress) has phosphatidylserine?
Outer
Associators of lipid-linked membrane proteins (and the actual link themselves) can be modulated, but that of integral membrane proteins can not. Why?
Integral membrane proteins typically cannot be easily altered because of how these proteins are attached to the membrane and the flexibility of their interactions
Lipid linked membrane proteins can be modulated because they are peripherally associated with the membrane via covalent attachment to lipids rather than being embedded within the lipid bilayer like integral membrane proteins
If a transmembrane protein has disulfide bonds – which side of the membrane are they on?
Always on the extracellular side
Are all beta barrels formed from the same number of strands?
No
They must be 10 amino acids long to span a membrane, but they can differ in size (circumference) with different numbers of beta strands
Do all beta barrels function as pores for transport?
No
Some bacteria use them as pores for passive diffusion channels
Other organisms use them for active transport, efflux systems, and secretion pathways
Under what condition might a membrane protein be completely unmovable within the membrane?
When anchored to the cytoskeleton or extracellular matrix, or when trapped within a microdomain or lipid raft
Under what condition might a membrane protein be freely movable but only within a subarea of the cell’s surface area?
When in an area partitioned by tight junctions, or when linked with other membrane proteins in a complex
Under what condition might a membrane protein be highly movable across wide areas of the plasma membrane?
When the protein is not anchored to anything and the membrane is fluid and lacks tension
For a multi- pass transmembrane protein, how might two alpha-helices interact within the lipid Bilayer?
They can interact by hydrophobic interactions like soluble proteins but with added constraint of being embedded in the membrane; residues pack closely together, stabilizing the interaction via hydrophobic forces in a lipid environment
Ex: Glycophorin A; dimerizes through hydrophobic interactions between transmembrane helices
Stability can be enhanced with hydrogen bonds, salt bridges, or disulfide bonds
Why are P-type pumps considered to be unusual/rule-breaking ATPases?
They are the only pump class with temporary self-phosphorylation
Why do glucose uniporters not have a change in binding affinity when importing glucose?
Because they function through facilitated diffusion (aka passive transport), which relies on conformational changes, not binding affinity changes
The uniporter doesn’t release glucose due to a drop in affinity, but due to the pull of the concentration gradient
ABC transporters - How is MDR1 able to transport both hydrophilic and hydrophobic molecules?
It’s ligands can enter from either the cytosol (if they’re hydrophilic) or from the cytosolic leaflet (if they’re hydrophobic)
It has a large adaptable binding pocket, which can interact with polar and nonpolar molecules. ATP driven conformational changes allow it to transport a diverse range of substances
Is ATP production via ATP synthases direct or indirect?
Indirect
What would happen to SRP if a mutation occurs in the targeting sequence?
SRP would fail to recognize and bind to the signal peptide sequence, so the cell’s secretory and membrane proteins wouldn’t be properly localized to the ER and may accumulate in the cytosol
What would happen to SRP if a mutation occurs in the ribosomes?
SRP would be unable to bind to the ribosome, making it unable to halt translation, leading to nascent polypeptides folding prematurely as they’re being synthesized
What would happen to SRP if a mutation occurs in the the SRP’s receptor?
SRP wouldn’t be able to deliver the ribosome complex to the ER membrane, so the ribosome and newly synthesized proteins would remain in the cytosol. Secretory and membrane proteins wouldn’t be localized to the ER
What does SRP stand for?
Signal riboprotein particle
Why is SRP called a signal riboprotein particle?
Because it’s composed of both RNA and proteins
How does Sec61 maintain the calcium gradient across the ER?
It has a tight seal with the ribosomes that prevents the leaking of calcium ions (and other stuff, like ATP)
It opens selectively, only for polypeptide translocation
Are SRPs found only in eukaryotes?
No, they’re also in bacteria
Are all ribosomes permanently associated with the ER?
No. Some ribosomes are free in the cytoplasm. Also, most ribosomes aren’t permanently attached, as they bind to the ER transiently during protein synthesis then detach after
