Protein Transport II (Lecture 13)

Question 1

True or False: Matrix targeting sequences LACK negatively charged (Asp, Glu) amino acids

Answer 1

True

Question 2

How are proteins imported into mitochondrial matrix?

Answer 2

Precursor proteins synthesized on cytosolic ribosomes

Polypeptide kept unfolded by chaperones (Cytosolic Hsp70)

MTS binds to outer membrane import receptor near a rare “contact site” with the inner membrane

MTS inserted into translocon of the outer membrane (Tom)

Translocating protein moves through Tom and inserts into translocon of the inner membrane (Tim)

MTS removed by a matrix protease

Hsp70 ATP hydrolysis releases newly imported protein

Protein folds into its mature, active conformation within the matrix

Question 3

What are the three energy inputs required for mitochondrial matrix import?

Answer 3

Cytosolic Hsp70

Matrix Hsp70 anchored to the Tim44 protein
Analogous to BiP and Sec63 of post- translational translocation into ER

H+ electrochemical gradient (proton- motive force) across the inner membrane

Question 4

True or False: Proteins must be unfolded to translocate!

Answer 4

True

Question 5

True or False: Path A is

analogous to type-I membrane proteins inserted in ER membrane

Answer 5

True

Question 6

Path B contains ______ sequence

Answer 6

Oxa1-targeting

Question 7

True or False: Path C does not have MTS.

Answer 7

True

Question 8

True or False: Many mitochondrial outer membrane proteins have a β-barrel structure

Answer 8

True

Question 9

What structure inserts β-barrel proteins into the outer membrane?

Answer 9

sorting and assembly machinery

(SAM) complex

Question 10

The ______ in chloroplast contain enzymes that help fix CO2.

Answer 10

Stroma

Question 11

True or False: Import of proteins into the
stroma is similar to import
into the mitochondrial
matrix

Answer 11

True

Question 12

______ are internal interconnected membranous sacs that contain all the enzymes for photosynthesis

Answer 12

Thylakoids

Question 13

SRP-Depedent pathway

Answer 13

Same as post translational translcation but (SecY instead of Sec61)

Question 14

pH Dependant pathway

Answer 14

Folded and bound to cofactor in stroma

Tat (twin-arginine translocation) pathway:

Thylakoid-targeting sequence must have two closely spaced Arg residues
— Tat proteins oligomerize in membrane to create pores of variable size to accommodate folded protein
— Powered by electrochemical gradient across thylakoid membrane
— Thylakoid-targeting sequence is cleaved off

Question 15

______ sequences direct post-translational transport of the unfolded proteins through translocons into the organelles

Answer 15

Targeting

Question 16

______ contain enzymes that oxidize amino acids and fatty acids, breaking them down for biosynthetic pathways

Answer 16

Peroxizomes

Question 17

True or False: Peroxisomes also contain catalase to

convert H2O2 into H2O

Answer 17

True

Question 18

True or False: All lack DNA and ribosomes (unlike

mitochondria and chloroplasts)

Answer 18

True

Question 19

True or False: Ubiquitination controls Pex5 recycling back into monomeric, soluble form in cytosol

Answer 19

True

Question 20

Peroxisomal proteins

Answer 20

Requires a C-terminal Ser-Lys- Leu targeting sequence (PTS1)

PTS1 binds to Pex5 receptor

Question 21

True or False: Peroxisomal membrane proteins are not targeted by PTS1 sequences

Answer 21

True, some (Pex3, Pex16) are placed in ER membrane

Question 22

______ allow movement of mRNAs, tRNAs, ribosomal subunits out of nucleus

Answer 22

Nuclear pores

Question 23

True or False: In nuclear pore complexes, Ions, small metabolites, proteins < 40 kDa can diffuse passively through NPCs (in or out)

Answer 23

True

Question 24

FG-nucleoporins are:

Answer 24

— Rich in hydrophobic Phe and Gly (F&G) residues (FG- repeats)
— Line the channel of NPC and associated with nuclear basket and cytoplasmic filaments
— Thought to form a gel-like matrix that allow diffusion of small molecules while excluding large ones

Question 25

True or False: Import of nuclear proteins requires nuclear-localization signals

Answer 25

True

Question 26

Targeting of nuclear proteins require ______, ______ and ______.

Answer 26

Nuclear-localization signal (NLS)

Ran

Nuclear transport receptor