Cell Bio 9 Flashcards
Biomembranes (2 Functions)
Define what is a cell
Allow for specialized functions to occur in a local manner
Biomembrane Components
Lipids
Sterols
Proteins
Due to ______, phospholipids spontaneously…
Amphipathicity, phospholipids spontaneously form lipid bilayers in acqueous solutions
Properties of Fatty Acids will
Confer properties onto the bilayers
Proteins Give…
Many functional characteristics to the membrane
Fatty Acids
Building blocks of the components of the membrane (phospholipids; sphingolipids)
Long hydrocarbon tails attached to a polar (variabled) carboxyl head group.
Amphipathic
Often Cx;y, where x= number of carbon molecules, y = number of double bonds
No double bonds
Saturated
One double bond
Unsaturated
More than one double bond
polyunsaturated
Melting Point
Increases with the length of the chain and decreases with the number of double bonds.
Length of chain increases the number of interactions.
Double bonds introduce kinks in the chain preventing tight packing of lipids against one another.
Membranes need to be
Fluid, not a solid or liquid at any temperature.
Adjust chain length, units of unsaturation and cholesterol such that the membrane cn be the right consistency at a certain temperature.
Properties of Biomembranes
Fluid
Closed Compartments
Semi-Permeable
Asymmetric
Fluid
Two dimensional fluids:
Rapid lateral diffusion
Rare transeverse (flip-flop) movement between leaflets
Fluidity is composition dependent.
Determinants of Fluidity
Fatty Acids length+saturation
Steroids: cholesterol can increase or decrease fludity
Proteins: large and strcutural, they can be tethered to the cytoskeleton, fixing a spot and things have to move aorund them altering fludidity
Temperature: not a mechanism used in living cells
FRAP
Fluorescent recovery after photobleaching
Measurement of membrane fluidity.
Flurescently label the plasma membrane proteins and bleach a certain area of the fluorescence and look for recovery of fluorescence.
Measure fluorescence, before and after bleaching.
Determine %recovery
Recovery can only occur if the proteins laterally diffuse in or out of this bleached area.
Level of recovery is related to mobility in the plasma membrane, 50% recovery tells you 50% are mobile and the other half are not.
Closed Compartments
Cytosolic Face = internal face
Exoplasmic face = external
Same orientation even in vesicles
Semi-permeability
Small, apolar (hydrophobic) molecules can diffuse freely
Large, ions, polar, charged molecules cannot diffuse freely and need to be transported
Protein Asymmetry + Membrane Function
Phospholipid composition differes between leaflets
Carbohydrates are found exclusively on the exoplasmic face
Proteins are either embedded in the bilayer in a fixed orientation or associated with only one side.
Pumps are placed in the membrane in a symmetric way so that are always pumping the same way.
Membrane Protein Categories
Integral
Lipid-Linked
Peripheral
All asymetric
Proteins in the membrane
Give a lot of function to what cell do
Protein within the membrane can carry out biological functions
Integral
Several domains of the protein are embedded in the hydrophobic integral part of the membrane.
Cyroplasmic Domain
Transmembrane Domain
Exoplasmic Domain
Cytoplasmic Domain
Amino Acids such as Arg and Lysine
(+)vely charged AAs are near the cytosolic side to interact with the polar head groups.
Anchor the protein to the lipid bilayer, the charged amino acids prevent the protein form being pulled into the hydrophobic core
Transmembrane Domain
Hydrophobic
Secondary and tertiary structures span the lipid bilayer.
Alpha helix (20-25 AAs in length)
Beta Barrel
Exoplasmic Domain
Glycosylated
adding sugars to the charged polar groups adds more polarity preventing the protien orm slipping into the membrane
Lipid-linked proteins
Proteins anchored to membrane by lipophilic adducté
Do not enter the lipid bilayer, attach to different compenents of the lipid bilayer
Lipids linked to the extracellular face
Use phosphatidlinositol to form a GPI anchor.
GPI anchor: that protein is linked to the lipid bilayer on the exoplasmic side, can diffuse freely laterally in the membrane
Link the protein to existing components within the plasma membrane
Lipids linked to the cytosolic face
To acylate a protein to the inner lipid bilayer the protein needs to have an N-terminal glycine such, that the protein is linked to the plasma membrane.
Prenylation proteins has a cys residue on the c-terminus
Peripheral Proteins
Attached through non-covalent interactions:
Ionic interactions, hydrogen bonds
protein-protien interactions
van der Waals forces
cytoskeletal filaments can associate with bilayer through peripheral proteins (adaptors) as can ECM components.
Integral membrane proteins like cadherins and integrins can link to the cytoskeleton and they use a variety of peripheral proteins to bind the cytoskeleton.
Topogenic Sequences Types
N-terminal (cleaved) signal sequence
Stop-transfer/membrane anchor sequence (STA)
Signal-anchor-internal (uncleaved) sequence (SA)
Hydrophobic C-terminus
Topogenic Sequence
Sequence gives rise to specific shapes, amino acids fold into different shapes, as transaltion is occuring, topogenic sequences are recognized.
Specific topogenic regions gives rise to a specific shape.
Membrane Protein Types
Type 1: Exoplasmic amino terminus
Type 2: Exoplasmic carboxy terminus
Type 3: Short exoplasmic amino terminus
Tail Anchored protein: short exoplasmic carboxy terminus
Type 4: transmembrane domains, exoplasmic amino terminus
GPI anchored proteins: Exoplasmic amino terminus+ carboxy terminus linked to GPI
Tail Anchored Proteins
Requires Get3 recognition of hydrophobic C-terminal tial, membrane embedded Get1 and Get2 + ATP hydrolysis.
Steps:
- This protein has a carboxy hydrophobic tail that gets recognized by GET3
- GET3 takes it to the ER membrane where GET1 and GET2 are waiting, through ATP hydrolysis this hydrophobic tail region is inserted into the membrane
- transmembrane protein without a luminal extracellular domain
Type 1 Proteins
Have N terminal signal sequence
Have stop transfer membrane anchor (STA): 2 topogenic sequence
One transmembrane domain luminal N-terminus
Steps:
1- Translation starts off in the cytosol; ribosome and mRNA are guided to the translocon by the SRP
2- Ribosome inserts the peptide into the translocon into the ER lumen
3- N-terminal domain is always luminal
4- The n-terminal signal sequence is cleaved by a peptidase.
5- STA topogenic sequence stops the ribosome for transferring the protein into the ER and anchors it into the membrane, the STA sequence becomes the transmembrane portion of the protein
GPI-anchored proteins
Begin as Type-1 proteins with N-terminal in lumen and a C-terminal STA
AA sequence near membranes recognized by GPI-transamidase, which cleaves and transfers the luminal portion to an adjacent GPI
Without a transmembrane domain, not a cytoplasmic domain and can bind to the cytoskelton, GPI anchored proteins have lateral mobility.
Type II protein
Have one internal signal-anchor sequence (SA)
Orientation determined by positively charged amino acids (kept in cytosol)
II= NH3 in cytosol
Ribosome and mRNA are in the cytoplasm, translation starts and the ribosome is moved to the ER membrane.
The C-terminal domain get put through the translocon and into the lumen because the positively charged amino acids are adjacent to the N terminal.
because of these charged amino acids its difficult to get the amino acid chain into the translocon,
Type III Proteins
NH3 is the lumen.
Positively charged amino acids are adjacent to the carboxy terminus
(+)vely charged amino acids are in the cytosol
Type IV Proteins
Orientation of intial helix determined by positively charged amino acids next to signal-anchored (SA) sequence.
Have alternating SA sequences and STA sequences
Can have even or odd number transmembrane domains
type 4 proteins are going to be inserted into the membrane, where the ribosome is constantly translating through the locon and through the cytosol.
