import/export of the nucleus

Question 1

What type of protein trafficking occurs with the nucleus?

Answer 1

Gated transport using nuclear pores.

Question 2

examples of cells with no nucleus and multiple nucleus

Answer 2

no nucleus: platelets, mature RBCs, some cells in lens of eye
multiple nucleus: skeletal muscle fibers, early drosophila embryos

Question 3

important features of the nuclear envelope

Answer 3

is a double bilayer, continuous with ER bilayer
perinuclear space is continuous with ER lumen
nuclear lamina is a layer of intermediate filament proteins (lamins)
nuclear pores are gates in the envelope
nuclear pore complexes are an assemblage of proteins controlling transport

Question 4

nuclear pore structure

Answer 4

nuclear pores are large protein complexes that are selective (gated). Channel nucleoporins act as size-selection filter, nuclear basket it located on nucleus side, and cytosolic fibrils are on cytosolic surface.

Question 5

what are nucleoporins?

Answer 5

proteins that compose the nuclear pore complexes. each NPC contains a set of ~30 different kinds of proteins (there are repeats) and a total of 500-1000 nucleoporins

Question 6

what are cytosolic fibrils composed of and what is their function?

Answer 6

contains phenylalanine and glycine repeats (FG repeats) and are thought to be binding sites for nuclear import receptors.

Question 7

what molecules can diffuse through nuclear pores?

Answer 7

molecules less than 9 nm in diameter can freely diffuse. larger molecules must be actively and selectivity imported

Question 8

what molecules does the nucleus import and export?

Answer 8

1. nuclear resident proteins which are translated in the cytosol and must be in nucleus to function
2. transcription factor proteins must enter nucleus in order to bind DNA and regulate transcription
3. mRNA enters cytosol to be accessed by ribosomes

Question 9

What determines if a molecule will be imported into the nucleus?

Answer 9

Nuclear Import Signals contained in proteins are specific and bind to different nuclear import receptors

Question 10

what is a nuclear import adaptor protein?

Answer 10

a protein that is homologous to nuclear import receptors and may be required by the import receptor because only the adaptor protein is capable of binding cargo protein

Question 11

what are G-proteins?

Answer 11

GTP binding proteins. they are enzymes that bind and hydrolyze GTP to GDP, known as GTPases. They act as molecular switches that toggle back and forth between two different conformations depending on if GTP or GDP is bound

Question 12

what is a GAP?

Answer 12

proteins that accelerate GTP hydrolysis

Question 13

what is a GEF?

Answer 13

proteins that induce a GTPase to release its bound GDP

Question 14

two main types of G-proteins

Answer 14

1. heterotrimeric G proteins: consists of a heterotrimer fo three different proteins
2. monomeric G proteins: monomers that are evolutionarily related to the alpha subunit of trimeric G proteins

Question 15

where is Ran-GAP present? Ran-GEF?

Answer 15

Ran-GAP is only present in the cytoplasm

Ran-GEF is only present in the nucleus (bound to chromatin)

Question 16

What is Ran?

Answer 16

a monomeric G protein that is important as a nuclear shuttling protein

Question 17

Where are the concentrations of GTP and GDP most abundant?

Answer 17

GTP is more abundant in the nucleus where GEF is present to release Ran-GDP bound GDP and enable the binding of the more abundant GTP. GDP must be more abundant in the cytoplasm and this is why Ran-GDP is able to bring it into the nucleus.