Nuclear Transport

Question 1

What allows transport across the nuclear membrane? What is a major subclass of this? When are they created and inserted into the nuclear envelope?

Answer 1

Nuclear pores allow transport across the nuclear membrane. Nucleoporins (NUPs) is a major subclass of nuclear pore. Nuclear pores are created and inserted into the nuclear envelope during interphase.

Question 2

Scaffold proteins

Answer 2

Aid nuclear transport and often contain FG repeats

Question 3

During mitosis, nuclear pores ___.

Answer 3

Disassemble. But otherwise they’re very stable proteins.

Question 4

Transport through nuclear pore

Answer 4

• Free diffusion, slow diffusion, active transport
• Several general strategies for active transport:
  (1) Adaptor protein directly binds to cargo protein and it the molecule that binds to the nuclear pore recognition site, causing it to be taken into nucleus
  (2) Adaptor protein induces a conformational change in cargo protein revealing nuclear pore recognition site, causing it to be imported into nucleus
  (3) Multiple adaptor proteins bind to the cargo protein and one of the adaptor proteins has a nuclear pore recognition site

Question 5

Exportins vs. importins

Answer 5

Exportins take things out of nucleus, importins take things in

Question 6

Ran Cycle

• What is Ran?
• Describe cycle

Answer 6

• Ran is a GTPase that provides energy for translocation of other proteins. It is active in GTP state
  Cycle:
  (1) Beta and alpha-importin binds a protein with a NLS
  (2) Complex enters via nuclear pore
  (3) Ran-GTP binds beta-importin, relasing alpha-importin+protein
  (4) CAS and Ran-GTP bind alpha-importin, releasing protein
  (5) Ran-GTP+importin+CAS complex diffuses back into cytoplasm
  (6) Ran-GTP hydrolyzes to Ran-GDP, breaking Ran+importin+CAS complex
  (7) Ran-GDP binds NTF2, which returns it to the nucleus
  (8) Ran-GEF regenerates Ran-GTP from Ran-GDP

Question 7

mRNA is transported across the nuclear membrane as _____. Steps?

Answer 7

• messenger ribonucleoprotein complexes (mRNPs)
• In general:
  (1) RNA Pol II produces mRNA
  (2) mRNP is assembled
  (3) ** Export signal (NXF1/NXT1) is added
  (4) mRNP binds NUP (rate-limiting)
  (5) mRNA is transported across nuclear membrane. Basket domain works as a quality-control sieve.

Question 8

What is another way mRNA can exit the nucleus?

Answer 8

mRNA can also exit via budding triggered by an atypical protein kinase C

Question 9

What do basket domains do?

Answer 9

Basket domains organize chromatin such that frequently-transcribed genes are close to the pore.

Question 10

What are lamins and what do they do? Where do variants come from? What do mutations in lamin proteins give rise to?

Answer 10

Lamins are intermediate filiaments that act as adaptors to organize chromatin. Variants arise from alternative splicing. Lamin defects lead to various tissue-specific pathologies (e.g. skeletal or cardiac dystrophy, diabetes) that manifest differently in different tissues, suggesting that chromatin organization is the result of tissue specific interactions with different basket proteins.

Question 11

What are histones?

Answer 11

Proteins that bind to and organize DNA

Question 12

What is a nucleosome?

Answer 12

One unit of DNA and the histone octamer to which it binds

Question 13

Chromatin is made up of ___

Answer 13

Chromatin is made of nucleosomes as well as enzymes and RNA

Question 14

Heterochromatin: found where, characterstics?

Answer 14

Found at the periphery of the nucleus; tightly packed

Question 15

Euchromatin characteristics

Answer 15

Loosely packed and thus more easily transcribed