Lecture 13 - Compartmentalisation of the cell and protein sorting second one

Question 1

What is nuclear import ?

Answer 1

Nuclear import refers to the process by which proteins, RNA, and other molecules are transported from the cytoplasm into the nucleus of a cell.

Question 2

What is nuclear export ?

Answer 2

Nuclear export is the process by which molecules, such as RNA and proteins, are transported out of the nucleus and into the cytoplasm of a cell.

Question 3

What is the nuclear pore complex ?

Answer 3

The nuclear pore complex is a large protein complex that spans the nuclear envelope and serves as the gateway for the transport of molecules between the nucleus and the cytoplasm.

Question 4

How does nuclear import occur ?

Answer 4

Nuclear import relies on specific signals called nuclear localization signals (NLS) present on cargo molecules. These signals are recognized by importins, which mediate the transport of the cargo through the nuclear pore complex into the nucleus.

Question 5

How does nuclear export occur ?

Answer 5

Nuclear export involves the recognition of nuclear export signals (NES) on cargo molecules by exportins. The cargo is then transported through the nuclear pore complex from the nucleus to the cytoplasm.

Question 6

Can large macromolecules, such as ribosomes, enter the nucleus?

Answer 6

Yes, large macromolecules like ribosomes can enter the nucleus through a process called passive diffusion. However, the import of most macromolecules requires active transport mediated by importins.

Question 7

What are some examples of nuclear import cargo?

Answer 7

Examples of nuclear import cargo include transcription factors, RNA-binding proteins, histones, and ribosomal subunits.

Question 8

What is the role of nuclear transport in gene expression?

Answer 8

Nuclear transport is crucial for gene expression as it allows for the transport of transcription factors and regulatory proteins into the nucleus, where they can interact with DNA and regulate gene transcription.

Question 9

How is nuclear transport regulated?

Answer 9

Nuclear transport is regulated through various mechanisms, including the presence of specific transport signals on cargo molecules, the binding and release of cargo by importins and exportins, and the Ran-GTP gradient across the nuclear envelope. Additionally, post-translational modifications and signaling pathways can influence the activity of transport factors.

Question 10

What are the essential components for all vesicle transport formation?

Answer 10

GTPase

Adaptor proteins

Coat

Question 11

How is GTP activated in molecular switches ?

Answer 11

GDP inactive - Guanine nucleotide exchange factors
GEFs - > GTP active

GTP active -> GTPase Activating Proteins
GAPs - > GDP inactive

GDP is formed in the cytosol
GTP is formed in membrane associated
Ras is the founding member of this family of GTPases

Question 12

What is the role of Ran in nuclear transport ?

Answer 12

Ran is a small GTPase that plays a critical role in regulating nuclear transport. It exists in two nucleotide-bound forms, Ran-GTP and Ran-GDP, and the interconversion between these forms controls the directionality of transport. Ran-GTP promotes cargo release in the appropriate compartment (nucleus or cytoplasm) and facilitates recycling of transport factors.

Question 13

How are proteins imported into mitochondria?

Answer 13

Proteins destined for mitochondria are typically synthesized in the cytoplasm and contain specific targeting signals, such as mitochondrial targeting sequences. These signals are recognized by receptors in the mitochondrial outer membrane, initiating protein import. The proteins are then translocated across the mitochondrial membranes and delivered to their specific subcompartments.

Question 14

What is the role of the translocase of the outer membrane (TOM) complex in mitochondrial protein import?

Answer 14

The TOM complex serves as the main entry gate for proteins into the mitochondria. It recognizes the mitochondrial targeting sequences on incoming proteins and facilitates their translocation across the outer mitochondrial membrane.

Question 15

How are proteins imported into chloroplasts?

Answer 15

Proteins targeted to chloroplasts are synthesized in the cytoplasm and contain transit peptides, which act as targeting signals. The transit peptides are recognized by receptors at the surface of the chloroplasts, leading to the translocation of the proteins across the chloroplast membranes.

Question 16

How are proteins imported into bacteria ?

Answer 16

Bacteria use a different mechanism called the Sec-dependent pathway for protein import. In this pathway, proteins are synthesized with signal peptides that target them to the Sec translocase in the plasma membrane. The Sec translocase facilitates the translocation of the proteins across the membrane into the bacterial cytoplasm.

Question 17

What is the function of the Sec translocase in bacterial protein import?

Answer 17

The Sec translocase is a protein complex present in the plasma membrane of bacteria. It recognizes signal peptides on newly synthesized proteins and facilitates their translocation across the membrane into the bacterial cytoplasm.

Question 18

What is a COPII vesicle?

Answer 18

COPII vesicles are a type of transport vesicles that mediate the movement of proteins from the endoplasmic reticulum (ER) to the Golgi apparatus in eukaryotic cells.

Question 19

What is the function of COPII vesicles ?

Answer 19

T he main function of COPII vesicles is to facilitate the selective packaging and transport of proteins from the ER to the Golgi apparatus for further processing, sorting, and secretion.

Question 20

What is the composition of COPII vesicles ?

Answer 20

COPII vesicles are composed of protein coats made up of the inner layer proteins, Sec23/Sec24, and the outer layer proteins, Sec13/Sec31. These coat proteins assemble on the ER membrane to form the COPII coat.

Question 21

What is the role of Sar1 in COPII vesicle formation?

Answer 21

Sar1 is a small GTPase protein that plays a crucial role in the formation of COPII vesicles. When activated by GTP binding, Sar1 recruits the COPII coat proteins to the ER membrane, initiating the assembly of the COPII coat and the budding of vesicle.

Expression of Sar1GDP inhibits COPII formation

Question 22

How does cargo selection occur during COPII vesicle formation?

Answer 22

Cargo selection during COPII vesicle formation involves the interaction between cargo molecules and cargo receptors or adaptors. These receptors recognize specific sorting signals on the cargo proteins and recruit them to the sites of vesicle budding, where they are incorporated into the forming COPII vesicles.

Question 23

What happens to COPII vesicles after they bud from the ER?

Answer 23

After budding from the ER, COPII vesicles fuse with each other and then fuse with the cis-Golgi network, delivering the cargo proteins to the Golgi apparatus. The vesicles then disassemble, and the COPII coat proteins are recycled back to the ER for further rounds of vesicle formation.

Question 24

What are the minimal components required for COPII formation?

Answer 24

Sar 1 GEF is Sec12
Sec23/24
Sec 13/31
ATP and GTP

Question 25

What is co-translational translocation?

Answer 25

Co-translational translocation is the process in which proteins are translocated across the ER membrane while they are still being synthesized by ribosomes. The growing polypeptide chain enters the ER lumen or membrane through a proteinaceous channel called the translocon.

Question 26

What is the transcolon ?

Answer 26

The translocon is a protein complex located in the ER membrane that serves as a channel for the translocation of proteins into or across the ER. It consists of two main components: the Sec61 complex, which forms the aqueous pore, and associated accessory proteins.

Question 27

What is the role of chaperones in ER translocation?

Answer 27

Chaperones are proteins that assist in protein folding and prevent aggregation during ER translocation. They help nascent polypeptides maintain their structure and protect them from degradation until they reach their final destination within the ER.