Unit 5:Intercellular Compartments and Protein Sorting

Question 1

What is the evolution of internal membranes of eukaryotes?

Answer 1

precursors of eukaryotes believed to be organisms (like bacteria) with no internal membranes
plasma membrane carried out all membrane-related functions
endomembrane system thought to have evolved as invagination of plasma membrane
mitochondria and chloroplasts thought to have evolved as endosymbionts

Question 2

Why is the cytoskeleton important?

Answer 2

-cell shape
-cell motility
-movement / position of organelles
-movement of materials within cell
-movement of chromosomes during mitosis

Question 3

What does protein sorting mean?

Answer 3

-the transfer of proteins into compartments where they are needed

Question 4

Where does the synthesis of all proteins start?

Answer 4

-in the cytosol on free ribosomes

Question 5

Does protein transport require energy?

Answer 5

Yes

Question 6

What directs proteins to the right place?

Answer 6

-stretch of amino acids, located at N-terminus, 15-60 AAs long, that directs proteins to particular organelles
-signal sequences for nucleus, mito/chloro, peroxisomes or ER
-usually removed after sorting

-delete or transfer sequence to another protein=protein goes to wrong ‘address’

Question 7

Can small molecules freely pass through nuclear pores?

Answer 7

Yes

Question 8

Can large proteins pass though nuclear pores?

Answer 8

No, the passage of large proteins is active
proteins pass through nuclear pores without unfolding

Question 9

What is the nuclear localization signal?

Answer 9

• amino acid sequence that ‘tags’ a protein for import into the nucleus by nuclear transport

Question 10

what does the nuclear export signal do?

Answer 10

tags a protein for export

Question 11

What moves out of the nucleus?

Answer 11

mature, properly processed mRNA
assembled ribosomal subunits (rRNA)

Question 12

What moves into the nucleus?

Answer 12

histones, ribosomal proteins, proteins required for transcription & DNA replication
dNTPs, rNTPs

Question 13

What are modifications done by the ER?

Answer 13

-formation of disulfide bonds
-stabilization
-addition of sugar groups
-protect
-keep in ER
-direct
-cell-cell recognition

Question 14

What happens if protein production exceeds capacity?

Answer 14

if protein prod’n exceeds capacity to keep up with folding
- misfolded proteins accumulate
-signals lead to increased expression of chaperone proteins and other proteins that assist in folding, expansion of the ER …
-if cell still can’t keep up, UPR will trigger cell death= apoptosis

Question 15

What do transport vesicles do with proteins?

Answer 15

-transport vesicles carry soluble proteins (in their lumens) and membrane proteins (in their membranes) between compartments

in general vesicle traffic is …
outward from ER =>Golgi =>other organelles? plasma membrane?
-inward, from plasma membrane => lysosomes

Question 16

What happens in the Golgi apparatus in the secretory pathway of protein?

Answer 16

series of flattened sacs – cisternae
organized into functionally distinct compartments with cis (entry) face closest to ER, trans (exit) face at other end
cis – newly formed
trans – breaking away
modification of new proteins arriving from ER:
peptide chains shortened by proteases
amino acids modified
CHO groups that were added in ER modified or removed
glycosylation
different CHO groups added to different AAs (ser, thr)
“O-linked glycosylation”

most complex polysaccharides synthesized in Golgi
glycos amino glycans in extracellular matrix (animals)
pectins, hemicellulose (plant cell walls)

Question 17

Constitutive secretion

Answer 17

-constant recreation
-unregulated

Question 18

What happens to the plasma membrane during secretion?

Answer 18

The plasma membrane gets bigger

Question 19

What happens in the endocytic Process(inward)

Answer 19

taking substances into cell by surrounding them with membrane
they become a membrane-bound vesicle
2 main types, based on size of vesicles formed:
pinocytosis ‘cell drinking’ – tiny vesicles formed - endosomes
done by all eukaryotic cells
phagocytosis ‘cell eating’ – much larger vesicles – phagosomes
only done by specialized cells

one more type in animal cells:
receptor-mediated endocytosis
very selective concentrating mechanisms
requires specialized receptors

Question 20

What is pinocytosis?

Answer 20

pinocytosis “drinking”
solutes, macromolecules, fluid
‘bulk’ – any molecules present in enclosed fluid enter cell

Question 21

What is phagocytosis?

Answer 21

phagocytosis “eating”
particles, other cells, debris

Question 22

What is receptor-mediated endocytosis?

Answer 22

-particular molecules (ligands) for which membrane has receptors
receptors grouped in patches of membrane called coated pits

Question 23

If a protein does not have a location signal where does it end up?

Answer 23

cytosol

Question 24

Where do all proteins start synthesis?

Answer 24

cytosol by free ribosomes

Question 25

What happens to the membrane associated with a vesicle that has exocytosed by its cargo?

Answer 25

the vesticle membrane fuses with the plasma membrane and becomes part of it

Question 26

How many membranes does the mitochondria have?

Answer 26

2

Question 27

How many membranes does a chloroplast have?

Answer 27

three

Question 28

What happens when a protein is needed in the chloroplast or mitochondria

Answer 28

-signal sequence at N terminus
-proteins must be moved across both outer and inner membranes at special sites where layers are in contact

Question 29

What steps are involved in protein import?

Answer 29

-proteins must be unfolded to be imported
-upon import must be folded and have the signal sequence removed

Question 30

What are the two types of proteins transferred to the ER?

Answer 30

-water soluble proteins translocated completely across into ER lumen
-destined for secretion, or lumen of an organelle
-prospective transmembrane proteins translocated only partially across
-destined for plasma membrane, ER membrane or membrane of another organelle

Question 31

What do temporary vesicles do?

Answer 31

-allow material to leave and enter cells
-move material between endomembrane compartments
-carry soluble proteins (in their lumens) to the plasma membrane for secretion
-move membrane proteins (in their membranes) to be expressed on the cell surface