Week 2: Membrane Structure and Proteins

Question 1

what % of the lipid bilayer do proteins vs lipids make up?

Answer 1

on average 50/50

Question 2

how wide is the lipid bilayer?

Answer 2

5 nm

Question 3

which types of lipids are the most abundant in membranes? what about animal cell membranes?

Answer 3

phospholipids; phosphoglycerides (1), sphingolipids, and sterols

Question 4

what type of phospholipid is phosphatidylserine? What type is sphingomyelin? what type is cholesterol?

Answer 4

a phosphoglyceride; a sphingolipid; a sterol

Question 5

what bonds do hydrophilic molecules make?

Answer 5

hydrogen bonding

Question 6

what is a micelle?

Answer 6

a cone shaped lipid bunch that has the hydrophobic tails on the inside and hydrophilic heads on the outside => spontaneously forms

Question 7

whats the most energetically favorable arrangement for phospholipids?

Answer 7

a bilayer due to the cylindrical shape of their 2 fatty acid tails

Question 8

what is a liposome?

Answer 8

a synthetic lipid bilayer made in a lab for experimental purposes that can be 25 nm to 1 micron in diameter

Question 9

where does phospholipid synthesis take place?

Answer 9

the cytosolic monolayer of the ER where lipids are made

Question 10

what does scamblase do?

Answer 10

non selectively equilibriates phospholipids between the 2 monolayers in the ER membrane to result in even growth
- flips from cytosolic side to lumen side

Question 11

what factors affect bilayer fluidity?

Answer 11

temperature, fatty acid chain length, double bonds, cholesterol

Question 12

lipid rafts

Answer 12

regions of a membrane enriched with sphingolipids, glycolipids, cholesterol, GPI anchored proteins, and some transmembrane proteins to form a micro domain of interactions
- patches of specialized structures and functions

Question 13

what is a glycolipid?

Answer 13

when fatty acid tails are linked to a glycerol head that protrudes out of the bilayer => does not have a protein region

Question 14

what is a glycoprotein?

Answer 14

a protein that has glycogen (sugar) on the head of its extracellular leaflet

Question 15

what is a GPI anchored protein?

Answer 15

a peripheral protein linked to a GPI lipid anchored head in the extracellular leaflet

Question 16

what is an oligosaccharide linker?

Answer 16

the sugar portion of the GPI linker between the lipid anchored part and the peripheral protein

Question 17

what does seipin do?

Answer 17

its a transmembrane protein that associates with assembly factors to help produce a budding lipid droplet => assembly factor stays on the droplet with associated proteins and seipin stays in the membrane on the cytosolic side

Question 18

what is phosphatidylserine?

Answer 18

a negatively charged molecule that stays on the inner cytosolic monolayer of the plasma membrane because it signals for apoptosis to macrophages

Question 19

where are glycolipids found?

Answer 19

exclusively found in the non cytosolic monolayer

Question 20

what does flippase do?

Answer 20

moves specific phospholipids from the outer monolayer to the inner monolayer in the plasma membrane

Question 21

why is asymmetry lost in apoptotic cells?

Answer 21

inactivation of flippase for phosphatidylserine and activation of scrameblase which transfers random phospholipids in both directions between monolayers

Question 22

what are gangliosides?

Answer 22

glycolipids containg one or more sialic residues (causes negative charge)

Question 23

what are functions of glycolipids?

Answer 23

protection of the membrane, cell recogntion and adhesion, sugar modification recognized by receptors, receptors for pathogens, etc.

Question 24

membrane protein classifications

Answer 24

transmembrane proteins that span the membrane or one leaflet, lipid linked proteins which attach to lipid anchors in the bilayer, peripheral proteins which noncovalently bind to transmembrane proteins

Question 25

hydrophobic residue

Answer 25

non-polar side chains

Question 26

glycophorin

Answer 26

determines ABO blood types in RBC's as a membrane spanning protein

Question 27

bacteriorhodopsin

Answer 27

has several membrane spanning domains and acts as a proton pump through light energy

Question 28

hydropathy plots

Answer 28

allow us to see transsmembrane amino acid sequences because they reach up to 1 on the graph

Question 29

beta barrel proteins

Answer 29

multiple membrane spanning strands of a polypeptide that form a sheet rolled up into a cylinder

Question 30

pourin

Answer 30

water filled channel made from a beta barrel protein in the plasma membrane

Question 31

where do lipid linked proteins attach? Examples?

Answer 31

some link to fatty acid chains, myristoyl, palmitoyl, prenyl groups associated in the cytoplasmic monolayer of the plasma membrane

Question 32

what does a GPI anchor do?

Answer 32

attaches to a peripheral membrane protein on the non cytoplasmic monolayer

Question 33

what is a signal switch?

Answer 33

occurs when modifications of a signal (usually extracellular) are reversed

Question 34

what are oligosaccharide chains?

Answer 34

found on glycosylated proteins on the extracellular side of the plasma membrane always

Question 35

disulfide bonds and where do they occur?

Answer 35

on the extracellular domains of proteins where cysteine amino acids have a S that attaches together - this doesnt happen on the cytosolic side because of the reducing environment

Question 36

glycocalyx

Answer 36

the carbohydrate rich zone on the surface of cells coated in sugars

Question 37

what contributes to the glycocalyx

Answer 37

glycoproteins, glycolipids, proteoglycans a type of glycoproteins, extracellular matrics adsorbed on the cell surface

Question 38

functions of the glycocalyx

Answer 38

protection, maintaining intercellular distances, cell recognition/adhesion

Question 39

what must you add for membrane proteins before isolation?

Answer 39

solubilization

Question 40

how do you isolate peripheral proteins

Answer 40

gentle extraction like high salt buffer, or altered pH

Question 41

how do you isolate GPI linked proteins

Answer 41

through the enzyme PI-PLC which digests the structure so the proteins can be released from the extracellular side (cleaves lipid anchor)

Question 42

what are detergents?

Answer 42

amphiphilic molecules that displace the phopholipids in the bilayer and isolate transmembrane proteins

Question 43

what was the cell called that fused mouse and human cells? what purpose did that serve?

Answer 43

heterocaryon; lateral diffusion of proteins

Question 44

what is FRAP?

Answer 44

fluorescent recovery after photobleaching => tells us how much/fast proteins diffuse in a membrane and if they may be restricted

Question 45

formation of membrane domains are from which 4 cases?

Answer 45

lipid rafts, interaction with the extracellular matrix, interaction with cell-cell adhesion, interaction with cytoskeleton

Question 46

what are the cytoskeletal networks made up of?

Answer 46

actin filaments bind the network together on the cytosolic leaflet of the plasma membrane

Question 47

what is a corral?

Answer 47

a domain enclosed by the cytoskeletal network that restricts lateral movements of membrane proteins

Question 48

what tracks protein diffusion via microscopy?

Answer 48

Signal particle tracking to define membrane domains