Lecture 13 - Overview and Nucleus - Intracellular Compartments and Protein Sorting

Question 1

What are the 3 main topological compartments of the cell?

Answer 1

• nucleus and cytosol
• organelles in secretory and endocytic pathways (ER, Golgi, endosomes, lysosomes)
• mitochondria

Question 2

What are the different mechanisms of protein trafficking?

Answer 2

• gated transport
• transmembrane transport
• vesicular transport

Question 3

What is gated transport? Which direction(s) can it occur in?

Answer 3

• active transport or diffusion through nuclear pore complex

- can occur from nucleus to cytoplasm or cytoplasm to nucleus

Question 4

What is transmembrane transport? Which direction(s) can it occur in?

Answer 4

• transport through protein translocators of specific molecules across membranes
• occurs only from cytosol across an organelle membrane

Question 5

What is vesicular transport? Which direction(s) can it occur in?

Answer 5

• transport between compartments via vesicles
• can occur in any direction between membrane bound compartments (in addition to cell exterior) excluding the mitochondria and nucleus

Question 6

What are characteristics of protein sorting signals?

Answer 6

• necessary and sufficient for protein targeting
• consists of a stretch of AAs
• characteristic of the AAs is more important than the specific AAs
• can be located on either the N terminus, C terminus, or within the protein
• may be removed by a signal peptides at destination
• recognized by complementary receptors

Question 7

What sort of sequences mark a protein for transport to into the nucleus?

Answer 7

-lysine and arginine (positively charged) rich sequence

Question 8

What sort of sequences mark proteins for import to the mitochondria?

Answer 8

-combination of positively charged AAs (arginine and lysine) in every 4th position and hydrophobic AAs

Question 9

What sort of sequence marks a protein to return to the ER?

Answer 9

-Lys-Asp-Glu-Leu (KDEL)

Question 10

What type of transport is nuclear transport and what molecules are transported?

Answer 10

• gated, bidirectional, and selective
• proteins needed in the nucleus are imported from cytosol
• tRNA, mRNA, and rRNA are exported to the cytosol

Question 11

What are nuclear pore complexes (NPCs)?

Answer 11

• pores connecting the cytoplasm with nucleoplasm
• allow for passive diffusion or facilitated transport in either direction
• composed of 30 different proteins (nucleoporins)
• octagonal symmetry
• protruding fibrils which particles bind
• basket structure

Question 12

What are nuclear localization signals (NLSs)?

Answer 12

• sorting signals rich in positively charged AAs, lysine and arginine
• target proteins for import into the nucleus

Question 13

What are nuclear import receptors (NIRs)?

Answer 13

• cytosolic proteins
• bind NLS and NPC fibrils
• phenylalanine/glycine (FG) repeat binding site
• move along FG repeats on NPC fibrils into the pore with cargo

Question 14

Describe the cycle of Ran nuclear import and export.

Answer 14

• cytosolic Ran-GDP enters the nucleus
• Ran-GEF (guanine exchange factor) exchanges GDP with GTP
• Ran-GTP binds import receptor causing it to release its cargo
• Ran-GTP is exported from the nucleus
• Ran-GAP cleaves a phosphate group from Ran-GTP making Ran-GDP