Lecture 11 & 12 (G proteins)

Question 1

Two main classes of signalling G proteins

Answer 1

• Heteromeric
• Monomeric (low molecular weight): Ras superfamily

Question 2

3 examples of Ras oncogenes

Answer 2

H-Ras, K-Ras, N-Ras

Question 3

What do Ras genes have in common?

Answer 3

• Encode proteins that are ~188 aa long
• Mw = ~20-25 kDa
• Have GDP and GTP binding activity

Question 4

Example of a point mutation in cellular Ras gene that changes it to an oncogene

Answer 4

Aspartic acid, valine or cysteine substituted for glycine at amino acid position 12 in Ras

Question 5

Point mutations in Ras lead to…

Answer 5

the protein being in a permanently active state (GTP-bound)

Question 6

Oncogenic mutations of Ras either…

Answer 6

(i) decrease the GTPase actvitiy of the Ras protein, or (ii) increase the rate of exchange of bound GDP for GTP

Question 7

Ras proteins are very frequently activated in __ cancers, particularly __ __

Answer 7

epithelial cancers
pancreatic tumours

Question 8

~85% of activated Ras in naturally occurring human tumours is __

Answer 8

K-Ras

Question 9

In recent years, some progress has been made developing drugs that target Ras __

Answer 9

G12C

Question 10

What does sos do?

Answer 10

It naturally converts the inactive form of Ras to its activated form (sos is a nucleotide exchange protein for Ras)

Question 11

5 sub-families of the Ras superfamily

Answer 11

• Ras
• Rho
• Rab
• Ran
• ARF (ADP-ribosylation factor)

Question 12

What is Rho important for?

Answer 12

regulating the actin cytoskeleton

Question 13

What is the largest sub-family of Ras?

Answer 13

Rab

Question 14

What is Rab involved in?

Answer 14

the control of vesicular trafficking

Question 15

What is Ran important for?

Answer 15

movement of proteins in and out of the nucleus

Question 16

Functional cycle of Ras proteins

Answer 16

Protein active when bound to GTP → protein conformation altered → interacts with effectors → downstream effects → once GTP is hydrolysed and the terminal phosphate is released, the protein returns to its GDP-bound inactivated state

Question 17

What does Ras-GAP do?

Answer 17

Stimulates the GTPase activity of the G protein to allow Ras to hydrolyse the terminal phosphate from GTP

Question 18

3 accessory proteins that convert Ras back into its activated state

Answer 18

• GDI (GDP dissociation inhibitor)
• GEF (nucleotide exchange protein)
• REP

Question 19

Ras proteins are crucial in…

Answer 19

growth factor signal transduction

Question 20

Domains within Ras proteins that are important for guanine nucleotide binding

Answer 20

G boxes

Question 21

Which region of G proteins is quite variable

Answer 21

C-terminal region

Question 22

C-terminal cysteines of the Ras protein are the sites for…

Answer 22

covalent addition of fatty groups like isoprenoids and palmitoyl groups to facilitate membrane association

Question 23

Where are Ras proteins typically located?

Answer 23

on the plasma membrane

Question 24

Mammalian Ras has been shown to function upstream of a group of kinases called the…

Answer 24

mitogen activated protein (MAP) kinases

Question 25

Normally, Ras is activated as a consequence of stimulation of __

Answer 25

RTKs

Question 26

Do GAP proteins negatively or positively regulate G proteins?

Answer 26

negatively

Question 27

Example of a GAP that was separately identified as a tumour suppressor gene

Answer 27

NF-1

Question 28

PTMs of C-terminal CAAX motif that allow monomeric G proteins to tightly associate with membranes

Answer 28

• Farnesylation (isoprenoids)
• Cleavage of -AAX
• Methylation
• Palmitoylation

Question 29

Best understood growth factor signal transduction pathway

Answer 29

EGF/Ras pathway

Question 30

Ras directly interacts with __ , and this event is dependent on…

Answer 30

Raf
Ras binding GTP

Question 31

Grb2 protein complexes with __

Answer 31

sos

Question 32

Two major pathways downstream of Ras activation

Answer 32

• MAPK
• PI3K

Question 33

Ral-GEF pathway is activated __ of Ras

Answer 33

downstream

Question 34

PI3 kinases play an important role as mediators of…

Answer 34

Ras-mediated cell survival and proliferation

Question 35

PI3K converts __ into __

Answer 35

PIP2 into PIP3

Question 36

PIP3 binds the __ domain of Akt (PKB)

Answer 36

PH

Question 37

PI3K can also activate __

Answer 37

Rac (Rho family of Ras superfamily)

Question 38

2 major effector proteins for Ras

Answer 38

Raf and PI3K