Lecture 9: The Nuclear Pore

These flashcards cover the essentials of BIS225 Lecture 10, focusing on cell compartmentalisation, nuclear transport, ER protein sorting, and organelle translocation.

What are the two main reasons for the compartmentalisation of the cell?

It allows for specialisation and increases efficiency (e.g., expansion of the ER).

According to Figure 12-5, what are the three primary types of intracellular protein transport?

Gated transport, transmembrane transport (from site of synthesis), and vesicular transport (to destination).

What is required for protein sorting to act as an "address label"?

Signals are required for protein sorting.

How many different nucleoporins typically make up the nuclear pore complex?

The nuclear pore complex consists of multiple copies of approximately $30$ different nucleoporins.

What is the structural composition of each nuclear pore complex?

Each complex appears to be made of $8$ subunits with a central plug.

Which molecules are transported from the cytoplasm to the nucleus for DNA synthesis?

Histone molecules, which are needed to package new DNA.

Which ribosomal subunits formed in the nucleolus must enter the cytoplasm?

The $60s$ and $40s$ subunits.

What is an advantage of separating transcription and translation regarding mRNA molecule usage?

It allows for the repeated use of one mRNA molecule, which increases efficiency.

What is the difference between the transcriptome and the proteome?

The transcriptome refers to what mRNA are present, while the proteome refers to what proteins are present.

Up to what molecular weight can molecules enter the nucleus by free diffusion?

Molecules with a weight of up to $5000$ are freely diffusible.

What is the molecular weight threshold at which a substance cannot enter the nucleus by diffusion?

$60,000$ daltons.

Under appropriate signal, what is the maximum diameter the nuclear pore can open up to?

$26\,nm$.

Which amino acids are typically found in nuclear transport recognition sites?

They are rich in Lys, Arg, and Pro.

What is the specific peptide sequence of the T antigen of SV40 virus important for nuclear transport?

Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val.

What temperature-based evidence supports that RNA transport is an active process in cells?

Transport of RNA is inhibited at $4^{\circ}C$.

In vitro, what is required for a fluorescently labelled protein to enter the nucleus?

$ATP$ is required; without it, the protein binds to the nuclear pore but remains outside the nucleus.

Who won the Nobel prize for the signal hypothesis regarding protein translocation into the ER?

Gunther Blobel.

What are the two ways newly made proteins can be translocated into organelles?

Co-translationally or post-translationally.

Where do ribosomes sit during co-translational translocation into the ER?

They sit very closely to the ER membrane and are not free in the cytosol.

What enzyme cleaves the signal peptide from the growing polypeptide chain in the ER?

Signal peptidase.

What specific sequence anchors a mature single-pass transmembrane protein in the ER membrane?

A hydrophobic stop-transfer sequence.

What characterizes the Type II topology of a single-pass transmembrane protein?

The N-terminus is located in the cytosol/cytoplasm.

What is the function of chaperones such as $BiP$ in the ER?

They ensure proper protein folding and prevent proteins from leaving the ER until they are folded correctly.

Which disease is caused by a defect in the folding of the CFTRdelta508 protein?

Cystic Fibrosis.

What response is stimulated by the presence of misfolded proteins in the ER?

The Unfolded Protein Response ($UPR$).

What can happen to a cell if the Unfolded Protein Response signal continues indefinitely?

It can lead to apoptosis.

Through which two complexes are proteins delivered into the mitochondrial matrix?

The TOM complex and the TIM23 complex.

What is the secondary structure of the mitochondrial N-terminal signal sequence?

An amphipathic alpha helix.

What bacterial complex is analogous to the mitochondrial SAM complex for outer membrane insertion?

The BAM complex.

Which complexes allow protein translocation across the chloroplast outer and inner membranes respectively?

The TOC complex (outer membrane) and the TIC complex (inner membrane).