Lec 08: Membrane Proteins and Transport

Question 1

What are some ways membrane proteins can associate with other membranes? (Figure 10-17)

Answer 1

1. Integral/transmembrane proteins 2. Peripheral membrane proteins 3. Lipid anchored membrane proteins 4. Membrane associated proteins

Question 2

Polypeptide chain of transmembrane proteins

Answer 2

often cross membrane as a-helix

Question 3

Cytosolic side chains of transmembrane polypeptide

Answer 3

-non-polar: gly, phe, leu, ala, thr (cytosol) -polar: his

Question 4

extracellular space side chains of transmembrane polypeptide

Answer 4

-non-polar: ile, ala, cys, gly, phe, ala -polar: his, ser, tyr

Question 5

Polarity of peptide bonds in transmembrane proteins

Answer 5

polar (number of hydrogen bonds are maximized in a-helix)

Question 6

How can transmembrane helices be predicted?

Answer 6

amino acid sequence (hydropathy plot)

Question 7

Mechanism of membrane spanning helices

Answer 7

-helix bending = loss of hydrogen-bonds ((exception: helices don’t contact lipids)

Question 8

Transmembrane B-barrel

Answer 8

-proteiens crystallize readily -relatively rigid

Question 9

Lipid anchors mediate?

Answer 9

Lipid anchors (fatty acid chain) mediate membrane association of some proteins

Question 10

3 lipid anchors

Answer 10

1. myristoyl 2. palmitoyl 3. farnesyl

Question 11

myristoyl anchor

Answer 11

amide linkage between terminal amino group and myristic acid

Question 12

palmitoyl anchor

Answer 12

thioester linkage between cystein and palmitic group

Question 13

farnesyl anchor

Answer 13

-prenyl chain thioether linkage between cysteine and prenyl group

Question 14

The membrane association of many signaling proteins is controlled by?

Answer 14

lipid anchors

Question 15

Src family kinases lipid anchors

Answer 15

-are myristoylated → This anchor alone mediates only weak membrane association → Kinase activation results in addition of a palmitic acid to cysteine

Question 16

Rab GTPases (lipid anchors)

Answer 16

-lipidated -C-terminal cysteines are modified with 1-2 geranylgeranyl anchors → Anchors are displayed in GTP-bound, but not GDP-bound state

Question 17

How are Extracellular moieties of proteins modified?

Answer 17

→ Glycosylated → Formation of disulfide bridges

Question 18

How do glycosylated extracellular moieties of proteins modified?

Answer 18

highly varied sugar modifications that mediate specific cell-cell interactions

Question 19

glycocalyx formed by?

Answer 19

• Glycosylated lipids and proteins (glycoproteins and proteoglycans) - Adsorbed glycoproteins (compounds of extracellular matrix)

Question 20

glycocalyx function

Answer 20

-protect against mechanical/chemical insults (low pH, digestive enzymes) -prevents unappropriated cell-cell interactions

Question 21

How do membrane proteins move?

Answer 21

-directional (coupled to active process) -diffusion (rotational/ lateral)

Question 22

Membrane proteins can be

Answer 22

• pack in crystal-like aggregates • confined by intercellular integrations

Question 23

Membrane proteins and lipids confined to?

Answer 23

• apical and basolateral domains
• lipid rafts
• by interactions - extracellular (e.g. matrix) - intracellular (e.g. with cytoskeleton)

Question 24

cortical cytoskeleton

Answer 24

-reinforces the plasma membrane mechanically (genetic abnormalities in spectrin cause anemia) → restricts diffusion of membrane molecules

Question 25

cortical cytoskeleton is rich in ?

Answer 25

actin

Question 26

dynamic remodeling of actin in cortical cytoskeleton

Answer 26

→ cell motility → endocytosis → transient membrane structures (e.g. filopodia)

Question 27

How does cortical cytoskeleton restrict diffusion of membrane molecules?

Answer 27

closely apposed to plasma membrane → restricts molecule motion (transiently or permanently) → plasma membrane is divided into corrals, that function by concentration proteins, such as signaling proteins

Question 28

How is cell membrane shape established and maintained?

Answer 28

→ pushing and pulling forces → membrane-bending proteins → Clustering of particular lipids - Phoshoinosides - Lipids subjected to phospholipase

Question 29

Membrane-bending proteins

Answer 29

→ that insert hydrophobic moiety only in one membrane leaflet → proteins that bind to membrane (Example: Clathrin coat) -lipid anchor

Question 30

How do membrane-bending proteins insert hydrophobic moiety only in one membrane leaflet?

Answer 30

hydrophobic protein domain (Example: Reticulons in ER tubule)

Question 31

Membrane transport proteins

Answer 31

→ are abundant → are multi-pass transmembrane proteins → are specific

Question 32

2 classes of membrane transport proteins

Answer 32

1) transporters 2) channels

Question 33

2 types of membrane transporters

Answer 33

-carriers -permeases

Question 34

Direction of diffusion determined by?

Answer 34

electrochemical gradient

Question 35

Diffusion direction of uncharged solutes determined by?

Answer 35

concentration gradient

Question 36

Diffusion direction of solutes with net charge determined by?

Answer 36

1. concentration gradient 2. electrical gradient

Question 37

Rate of transporter-mediates vs channel-mediated diffusion (chart)

Answer 37

Channel--\> up to 10^8 molecules/second Transporter--\> 10^2-10^4 molecules/second

Question 38

Rate of transporter-mediates vs channel-mediated diffusion (chart)

Answer 38

Channel--\> up to 10^8 molecules/second Transporter--\> 10^2-10^4 molecules/second

Question 39

Transporters have a?

Answer 39

-Vmax -Km -can be inhibited (competitive or allosteric inhibition)

Question 40

Vmax

Answer 40

speed of conformational change

Question 41

Km

Answer 41

affinity for substrate

Question 42

Active membrane transport

Answer 42

powers transport of a molecule against the electrochemical gradient

Question 43

Three types of active membrane transport (figure)

Answer 43

- uniporter - symporter - antiporter

Question 44

Coupled transporter

Answer 44

-use electrochemical gradient of one molecule to pump another molecule -drives secondary active transport -works backwards like an enzyme

Question 45

Na+ symporters functionality

Answer 45

-cooperative binding -conformational change only possible when completely empty or fully occupied -thermal energy drives conformational change

Question 46

Na+ symporters

Answer 46

- transport molecules such as sugars, amino acids, and neurotransmitters - are abundantly expressed in intestine, kidney, and neurons

Question 47

Transporters are built from?

Answer 47

inverted repeats/functional symmetry due to pseudosymmetric conserved core generated by gene duplication

Question 48

transcellular transport enabled by?

Answer 48

asymmetric distribution of transporters across apical/basolateral membrane domains

Question 49

Microvilli can ?

Answer 49

increase absorptive membrane area up to 25 fold

Question 50

Tight junction function

Answer 50

permeability barrier for solutes and membrane proteins