MVU6 MEMBRANES - 1 Flashcards
end of the ubiquitination lecture and membranes
is it possible that a polypeptide sequence is not degraded by the proteasome?
yes, if it doesnt have lysines
how many proteins in total do the cutting in a proteasome?
6
what do AAA family ATPases do?
unfold proteins till primary structure
why do proteins need to be in primary structure before entering proteasome?
because the ring is very narrow
what is the function of the Ub receptor?
increases efficiency of targeting
on the outside of the cap
recognises the polyUb tail
selects only the K48 chains
protects against premature DUB activity
what are the two types of Ub receptors and what do they do?
- Intrinsic receptors: the cap subunit Rpn10 and Rpn13 bind polyUb
- extrinsic (shuttling) Ub receptors
separate from the proteasome
bind polyUb through Ub-associated domain (UBA)
have a Ub like domain (UBL) that is recognised by the cap
recognise poly UB proteins and bring them to the proteasome
what is the unfoldase protein called?
Rpt 1-6
what is the DUB protein called?
Rpn8/11
what happens when the Ub receptors recognise the tail?
conformational change and the tail is removed
what happens in the proteasome core?
cavity inside is small and narrow
proteins have to stay unfolded
3 active subunits in each beta ring, 6 in total
one cuts basic AAs, one cuts at acidic AAs, and one cuts at hydrophobic AAs
what happens after the proteins are cut?
peptides diffuse out and are digested into amino acids by peptidases
what defines the cell?
the membrane
what is the secretory pathway?
a transport system between several types of organelles in the cell surface
synthesis of proteins and proteins at the ER
traffic through the GA, to the plasma membrane
internalisation through endosomes, to degradation in lysosomes
some organelles (mitochondria) are not connected in the secretory pathway
what are characteristics of the lumen of secretory organelles?
the interior of the lumens is continuous with each other and the extracellular space
the lumental environments are similar to the extracellular space (blood plasma) and different from the cytosol
what do vesicles do?
bud from one organellar membrane and fuse with another without releasing contents to the cytosol
closed till they reach the target
what type of environment is the cytosol?
reducing
what type of environment is the lumenal/extracellular fluid?
oxidising
what functions do biological membranes fulfill?
provide enclosure to cells and to organelles within cells
allow regulated transport of materials between compartments
provide sites within cells for biochemical reactions (accumulate certain types of molecules)
photosynthesis, oxidative phosphorylation, metabolism of biological molecules
support contacts with the environment outside cells (cell motion, cell recognise other cells, cell fusion)
transmission of signals from exterior to interior of cells
what are properties of membranes?
form hydrophobic barriers between aqueous compartments within cells
flexible and can be folded into different shapes
selectively permeable to small hydrophobic molecules, but not to larger or charged/polar molecules
specialised protein complexes control the movement of impermeable molecules across membranes
can store energy as concentration gradients
- voltage (nerve cells)
- pH, potassium, sodium, calcium gradients
what are membranes made of?
lipid molecules and membrane proteins
how are lipids organised?
into a bilayer, polar on each side and hydrophobic in the middle
what can membrane proteins do in the fluid bilayer?
membrane proteins can rotate and diffuse laterally in the bilayer
they cannot flip
what are the major lipids in membranes?
phospholipids - in all membranes
glycolipids - only at plasma membranes (not in ER or GA)
cholesterol
other types with special functions
all have polar and hydrophobic sections
what does lipid composition determine?
physical properties of the membrane
mobility (diffusion rotation) and curvature, thickness
how can lipids move in the membrane?
lateral diffusion
flexion
rotation
can flip (rarely occurs)
what are characteristics of phospholipids?
most abundant lipid
polar head groups:
has choline or other charged group
phosphate and glycerol (attaches tail to phosphate)
classification by head groups
2 fatty acid tails:
different lengths
saturated or unsaturated (makes a bend)
found in many different combinations with head groups
through what linkage is glycerol linked to the tail?
ester linkage
what are the most common phospholipid head groups?
phosphatidyl choline (PC)
-ethanolamine (PE)
-serine (PS)
what is special about phosphoserine?
the only one that has a negative charge
what is another related head group that is related but is not a glycerolipid?
sphingomyelin (SM)
has an amide linkage with the fatty acid tail
what are characteristics of phosphatidyl-inositol (PI)
not abundant but can be phosphorylated and act as a signaling molecule
has a sugar as a head (has hydroxyls, can be phosphorylated)
what does head group affect?
size and charge affect lipid mobility
what are characteristics of fatty acid tails?
14 to 24 carbons
varying number of double bonds
saturated tails are straighter and more flexible (increase length)
double bonds introduce bends in the tail, reduce flexibility and overall length
what do the types of tails in membranes determine?
thickness and fluidity of the membrane
what are characteristics of glycolipids?
only found on the outside surface of the plasma membrane
head groups contain different sugar groups in many combinations
important for cell contacts with environment and other cells
the sugar is a glucose, galactose or NANA sugar
what are the characteristics of cholesterol?
structurally different from other lipids
steroid ring structure makes it very rigid
lateral mobility, rotation is much lower
reduces mobility of surrounding phospholipids, makes fatty acids more rigid
amphipathic
integrated in the membrane
makes the PM thicker
interacts with tails through hydrophobic interactions
why is the asymmetry of the plasma membrane important for function?
exterior has glycolipids
interior has stronger negative charge (transmit signals) (high PS levels)
asymmetry is not absolute but is actively maintained
what is mostly present on the outside and inside of the membrane?
outer leaflet: PC, SM, glycolipids
inner: PE, PS, PC (low amounts of PI)
what are the different lipid compositions between the organelles?
PM has highest level of cholesterol, SM
ER and mitochondria have higher levels of PC and PE
other specialised lipids are found in different membranes
what are microdomains?
regions of a membrane that are organised laterally (sideways) in patches
PM and trans golgi have special mitochondria called lipid rafts
thicker than surrounding membrane, enriched in cholesterol
lipids with longer tails cluster together in rafts
cholesterol binding straightens lipid tails, causing thicker membrane
different protein content and biological function
