Nuclear Envelope and Molecule Transport

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Questions and Answers

What is the primary function of the nuclear lamina?

To transport molecules between the nucleus and cytosol

To replicate DNA

To provide structural support for the nuclear envelope (correct)

To synthesize proteins

What is the relationship between the outer nuclear membrane and the endoplasmic reticulum (ER)?

They are separate structures with no connection

The ER is a part of the outer nuclear membrane

The outer nuclear membrane is continuous with the ER (correct)

The outer nuclear membrane is a part of the ER

What is the purpose of the nuclear pore complexes?

To synthesize proteins

To replicate DNA

To provide structural support for the nuclear envelope

To conduct bidirectional traffic between the cytosol and the nucleus (correct)

What is the role of the transmembrane proteins in the inner and outer nuclear membranes?

To link the nuclear lamina to the cytosolic cytoskeleton Signup and view all the answers

What is the function of the ribosomes on the outer nuclear membrane?

To synthesize proteins Signup and view all the answers

What is the destination of the proteins made on the ribosomes on the outer nuclear membrane?

The perinuclear space Signup and view all the answers

What is the function of the proteins made on the ribosomes on the outer nuclear membrane?

To function in the nucleus Signup and view all the answers

What is the direction of protein transport in the perinuclear space?

Both directions Signup and view all the answers

What triggers the disassembly of the nuclear lamina during mitosis?

Phosphorylation of the lamins Signup and view all the answers

What is the result of dephosphorylation of the lamins during mitosis?

Reassembly of the nuclear envelope Signup and view all the answers

What is the site of lamin re-formation during mitosis?

Around regions of individual decondensing daughter chromosomes Signup and view all the answers

What is a key difference between mitosis in metazoan cells and yeasts?

The nuclear envelope breaks down in metazoan cells but remains intact in yeasts Signup and view all the answers

What is involved in the reassembly process of the nuclear envelope during mitosis?

Dephosphorylation of some nucleoporins and proteins of the inner nuclear membrane Signup and view all the answers

How do the membranes eventually form a complete nucleus during mitosis?

Through the fusion of membrane structures Signup and view all the answers

What is the function of nuclear localization signals?

To regulate the selectivity of active nuclear import Signup and view all the answers

What is the approximate number of NPCs in the nuclear envelope of a typical mammalian cell?

3000-4000 Signup and view all the answers

What is the main component of the central pore of an NPC?

Channel nucleoporins Signup and view all the answers

What is the purpose of the FG motifs in the NPC?

To regulate the diffusion of large macromolecules Signup and view all the answers

What is the main function of the nuclear envelope?

To regulate the selective transport of macromolecules Signup and view all the answers

What is the internal diameter of each NPC?

40 nm Signup and view all the answers

What is the approximate number of macromolecules an NPC can transport per second?

1000 macromolecules Signup and view all the answers

What is the characteristic feature of scaffold nucleoporins?

They are membrane-bending proteins Signup and view all the answers

What is unique about the NPC transport mechanism?

It can transport fully folded proteins and large multiprotein complexes. Signup and view all the answers

Where do particles first arrive during NPC transport?

At the tentacle-like fibrils of the scaffold nucleoporins Signup and view all the answers

What is the function of a nuclear localization signal (NLS)?

To impart the ability of large particles to navigate through the nuclear pore Signup and view all the answers

What happens to a protein with an altered nuclear localization signal?

It remains in the cytosol Signup and view all the answers

What is the role of adaptors in nuclear import?

To recognize nuclear localization signals on cargo proteins Signup and view all the answers

What is required for cargo protein 4 to bind to its nuclear import receptor?

An adaptor protein Signup and view all the answers

Where is the precise location of the signal within the amino acid sequence of a nuclear protein important?

Nowhere in the protein sequence Signup and view all the answers

What type of pore does the NPC transport mechanism use?

A constitutively open, mesh-filled pore Signup and view all the answers

What is the role of the nuclear localization signal in nuclear import receptors?

It is exposed only when the cargo protein is loaded, allowing the receptor to recognize it. Signup and view all the answers

What is the function of adaptor proteins in nuclear import?

They form a bridge between the import receptor and the nuclear localization signal. Signup and view all the answers

What is the composition of the unstructured domains of several nucleoporins?

FG repeats Signup and view all the answers

How do import receptors traverse the distance across an NPC?

By diffusion accelerated by the import receptor Signup and view all the answers

What is the role of the FG repeats in the nuclear pore?

They form a mesh that the import receptor must disrupt to enter the NPC. Signup and view all the answers

What happens to the import receptor after it has delivered its cargo to the nuclear compartment?

It returns to the cytosol to transport the next cargo protein. Signup and view all the answers

What is the mechanism by which the import receptor confers directionality to the import process?

By selectively dissociating the cargo protein only on the nuclear side of the NPC. Signup and view all the answers

What is the characteristic of the binding sites of nuclear import receptors for the FG repeats?

Low-affinity binding sites Signup and view all the answers

Study Notes

The Nuclear Envelope

The nuclear envelope is a double membrane structure penetrated by nuclear pore complexes.
The outer nuclear membrane is continuous with the endoplasmic reticulum (ER) and is studded with ribosomes engaged in protein synthesis.
The proteins made on these ribosomes are transported into the perinuclear space, which is continuous with the ER lumen.

Nuclear Lamina

The nuclear lamina is a meshwork of polymerized protein subunits (intermediate filament family of cytoskeletal proteins) called nuclear lamins.
The lamina provides structural support for the nuclear envelope and acts as an anchoring site for chromosomes and nuclear pore complexes.
The lamina is connected to the cytoplasmic cytoskeleton via protein complexes that span the nuclear envelope.

Nuclear Pore Complexes (NPCs)

NPCs are composed of approximately 30 different proteins, or nucleoporins.
NPCs display eightfold rotational symmetry and can transport a staggering 1000 macromolecules per second in both directions.
The internal diameter of each NPC is ∼40 nm, large enough to accommodate ribosomal subunits and even viral particles.
Nucleoporins can be classified into transmembrane ring proteins, scaffold nucleoporins, and channel nucleoporins.

Nuclear Localization Signals (NLSs)

NLSs are responsible for the selectivity of nuclear import and are typically rich in positively charged amino acids lysine and arginine.
NLSs can function even when linked as short peptides to the surface of a cytosolic protein.
NPC transport occurs through a large, constitutively open, mesh-filled pore, rather than through a smaller protein translocator.

Nuclear Import Receptors

Different nuclear import receptors bind different nuclear localization signals and thereby different cargo proteins.
Cargo proteins sometimes require an adaptor protein to bind to their nuclear import receptor.
Adaptor proteins are structurally related to nuclear import receptors and recognize nuclear localization signals on cargo proteins.

Nuclear Import Process

Nuclear import receptors recognize nuclear localization signals on cargo proteins and bind to them.
The receptor-cargo complex is recruited to NPCs through interactions with FG repeats on nucleoporins.
The receptor-cargo complex locally dissolves the gel-like mesh inside the NPC and can diffuse into and within the NPC pore.
The import receptor returns back to the cytosol for transport of the next cargo.

Mitosis and Nuclear Envelope Breakdown

Phosphorylation of the lamins triggers the disassembly of the nuclear lamina, which initiates the breakup of the nuclear envelope.
Dephosphorylation of the lamins reverses the process.
The lamin network begins to re-form around regions of individual decondensing daughter chromosomes during nuclear reassembly.

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Description

This quiz covers the structure and function of the nuclear envelope, including nuclear pore complexes and the transport of molecules between the nucleus and cytosol. Learn about the role of transmembrane proteins and the connection to the endoplasmic reticulum.