Nuclear Transport

Question 1

Describe the structure of the nuclear pore complex and roles of nucleoporins (Nups)?

Answer 1

The NPC has an 8-beam symmetry square to the membrane and is asymmetric to the membrane surface [44-47]. In a simplified way, the complex consists of three substructures: (1) the cytoplasmic filaments, (2) a central core, and (3) the nuclear basket (looks like a pipette tip)

Proteomic studies of mammalian and yeast NPCs showed that an NPC consists of about 30 different nucleoporin proteins whose copies are present in a large quantity (as a rule, 8, 16, or 32 copies as follows from the 8-beam symmetry), their total number being 500-1000 per NPC

Question 2

What are karyopherins? What is their role in transport?

Answer 2

Karyopherins are a group of proteins involved in transporting molecules between the cytoplasm and the nucleus of a eukaryotic cell. The inside of the nucleus is called the karyoplasm (or nucleoplasm). Generally, karyopherin-mediated transport occurs through the nuclear pore, which acts as a gateway into and out of the nucleus. Most proteins require karyopherins to traverse the nuclear pore.

Karyopherins can act as importins (i.e. helping proteins get into the nucleus) or exportins (i.e. helping proteins get out of the nucleus).

Question 3

What are nups? What is their role in transport?

Answer 3

“nucleoporins” (Nups) are 30 distinct proteins repetitively arranged in distinct sub-complexes (total of 500-1000 proteins/NPC).

Question 4

What are the major mechanisms by which nuclear transport can be regulated?

Answer 4

1. entropy barrier created by Nups (nucleoporins)
2. Ran GTP/GDP gradient
3. Interaction of cargo transporters with nucleoporins
4. cargo modifications that impact association with transporters

Question 5

What are two examples given in class of diseases caused by improper nuclear transport?

Answer 5

1. Beta-catenin (Normal location, cytoplasm—>to nucleus)

2. p53 (Normal location nucleus—->cytoplasm)

Question 6

What does NPC stand for?

Answer 6

Nuclear pore complex.

Question 7

Protein that must be imported to the nucleus from the cytoplasm carry ______ that are bound by _____.

Answer 7

nuclear localization signals (NLS); importins

[A NLS is a sequence of amino acids that acts as a tag. They are not cleaved during transport so can be re-used]

Question 8

Proteins, transfer RNA, and assembled ribosomal subunits are exported from the nucleus due to association with ____, which bind signaling sequences called _____.

Answer 8

exportins; nuclear export signals (NES)

They are not cleaved during transport so can be re-used

Question 9

The ability of both importins and exportins to transport their cargo is regulated by the small GTPase, _____.

Answer 9

Ran.

[Ran comprises 0.4% total cell protein ≈107 copies/cell]

Question 10

In the nucleus, Ran exists as _____. In the cytoplasm, it exists as _____.

Answer 10

RanGTP; RanGDP

Question 11

In its GTP bound state, Ran is capable of binding karyopherins (importins and exportins). Importins _____ cargo upon binding to RanGTP. Exportins ____ RanGTP to export their cargo.

Answer 11

release; bind

Question 12

Importin proteins bind their cargo in the cytoplasm, after which they are able to interact with the nuclear pore complex and pass through its channel.[2] Once inside the nucleus, interaction with Ran-GTP causes a conformational change in the importin that causes it to _____ from its cargo.

Answer 12

dissociate

Question 13

In the nucleus, the exportin binds the _____ and ____ and diffuses through the pore to the cytoplasm, where the complex dissociates.

Answer 13

cargo and Ran-GTP

Question 14

What may be considered the main functional unit of an NPC?

Answer 14

The FG repeats (Phy/Gly) are a class of Nup

[30% of Nups contain FG repeats. FG-nucleoporins have sites of binding to karyopherins. When passing through NPCs, all karyopherins bind to FG-nucleoporins. Just these interactions make the basis of the current model of nucleocytoplasmic transport [4]. It is assumed that asymmetrically located FG-nucleoporins play an important role in the transport directionality and the substrate release. Thus, importins often have a higher affinity to nucleoporins of the nuclear face of the NPC, and exportins to its cytoplasmic face]

Question 15

GTPase activating protein (GAP): stimulates hydrolysis of ___to ____. Is GAP Cytoplasmic or Nuclear?

Answer 15

GTP to GDP; cytoplasmic

Question 16

Nuclear Guanine exchange factor (GEF): removes ___ and replace with __. Is GEF Cytoplasmic or Nuclear?

Answer 16

GDP; GTP; nuclear