Protein sorting I

Question 1

why are post translational modifications necessary

Answer 1

regulate protein biological activity, location and interaction with other molecules

Question 2

where do PTM occur

Answer 2

at distant amino acid sites

Question 3

examples of PTM are

Answer 3

proteolysis, glycosylation, phosphorylation, ubiquitination

Question 4

Proteolysis

Answer 4

is irreversible. Is used for protein sorting and regulating protein activity

Question 5

glycosylation

Answer 5

oligosaccharide chains (glycan) are added to proteins in the ER and Golgi.
it can happen in 2 ways: N-glycosylation or O glycosylation

Question 6

N-glycosylation

Answer 6

occurs on asparagine residues

Question 7

O-glycosylation

Answer 7

occurs on serine, threonine residues

Question 8

what are the functions of glycosylation

Answer 8

correct folding of modified proteins and confer solubility, protect protein from proteolysis, may help cell adhesion, are used as surface signals

Question 9

phosphorylation

Answer 9

occurs at Ser, Thr, Tyr residues. Is used to switch on/off proteins

Question 10

ubiquitylation

Answer 10

ubiquitin is a small protein that can be covalently attached to other proteins on Lys residues.

Question 11

mono ubiquitylation

Answer 11

histone regulation

Question 12

multi-ubiquitylation

Answer 12

endocytosis

Question 13

poly-ubiquitylation

Answer 13

proteosomal degradation

Question 14

where do proteins go after synthesis

Answer 14

stay in the cytoplasm or be sorted to cell compartments

Question 15

when can protein sorting occur

Answer 15

before or after synthesis

Question 16

what are necessary for protein sorting

Answer 16

~localization signals: in the protein that must be sorted

~sorting receptor/translocator: guides protein to correct destination

Question 17

what are the 2 kinds of localization signals

Answer 17

~signal sequence: is a continuous stretch of AA

~ signal patch: separate AA motifs brought together when the protein folds

Question 18

note that

Answer 18

physical properties of he signal and location within a protein are more important than exact AA sequence

Question 19

proteins are sorted with 3 main methods

Answer 19

gated, transmembrane, vesicular transport

Question 20

what kind of transport is used to enter the nucleus

Answer 20

gated transport

Question 21

how do you enter the nucleus

Answer 21

• through nuclear pores
• small molecules can move by diffusion, macromolecules are transported by active diffusion
• proteins can be transported fully folded
• localization signal is not removed after sorting

Question 22

the nucleopore is

Answer 22

bidirectional , it imports histones, non histonic proteins, ribosomal proteins.
exports: mRNA and ribosomal units

Question 23

what molecules are actively transport through NPC

Answer 23

molecules >40kDa, is protein mediated, requires energy

Question 24

what molecules can freely diffuse through NPC

Answer 24

molecules <40kDa, and diameter<9nm

Question 25

NPC permeability declines with

Answer 25

age, due to deletion of some nucleoporin, and oxidative damage

Question 26

what are the 4 main substances involved in Nuclear transport

Answer 26

- proteins of NPC - nuclear localisation signal, nuclear export signal - nuclear receptors (karyopherins) - monomeric Ran GTPase

Question 27

Nuclear localization signals

Answer 27

are one or two short sequences rich in basic amino acids (Lys Arg)

Question 28

nuclear export signals

Answer 28

sequences rich in hydrophobic AA - Leu

Question 29

what are the 2 kinds of nuclear receptors

Answer 29

importins and exportins

Question 30

nuclear transport requires energy when

Answer 30

when transporting in either direction

Question 31

where does energy for nuclear pore transport come from

Answer 31

Ran GTPase which has 2 states. Ran GAP (GTP activating protein) and Ran GEF (guanine exchange factor). They make a conformational gradient

Question 32

explain nuclear import

Answer 32

- importin binds to cargo and NPC and transports it into nucleus - binding of RanGTP to importin causes cargo release - importin bound to RanGTP is transported back to cytosol - hydrolysis of GTP makes RanGDP dissociate from importin

Question 33

how does nuclear export work

Answer 33

- in the nucleus, exportin binds both RanGTP and cargo and transports it to the cytosol - hydrolysis of GTP makes Ran GDP dissociate from exportin and cause cargo release - free exportin goes back to nucleus