Intracellular protein transport

What is a NLS?

What detects NLS?

How is this useful?

Nuclear Localisation Signal - a stretch of 6 +vely charged residues in the primary sequence.

Importin, which allows direct passage either way though nuclear pores.

NLS sequences can be masked or unmasked by modifying a protein, allowing transport to be regulated

What amino acids are present in the sequence SKL-e?

What does the C-terminal sequence -SKL-e do?

Serine, lysine, leucine, (end)

It is recognised by the PTS1 receptor, and so lets the protein enter a peroxisome. This is irreversible.

What is co-translational targeting?

Movement of partially transcribed proteins

What sequence does the signal recognition particle detect?

How does the SRP act?

-ψψψψψψψψ- (at least 8 hydrophobic residues continuous)

Causes translation to arrest, SRP associates with an SRP receptor and adjacent translocon on the ER membrane, signal sequence is checked, orientation of peptide chain in translocon is set, SRP leaves.

What is a translocon?

How does it act?

Membrane spanning protein in the ER with a pore inside.

Receives nascent peptide from SRP, collects forming membrane spanning domains inside its pore. Once translation of all transmembrane domains is complete, the protein passes into the ER membrane.

What further modifications occur when a protein enters the ER lumen via the translocon?

When does this occur?

A protease cleaves the signal sequence if it was at the N-terminus, sugar residues are attached to the Asn in any -Asn-X-Ser- or -Asn-X-Thr-, chaperone proteins pull the growing chain into the lumen, disuphide bond formation is catalysed, etc.

During transcription

What happens when a damaged protein enters the ER lumen?

What happens when this mechanism is overactive?

ER-associated degradation. Protein is ejected form ER, ubiquitin ligase attaches ubiquitin molecules to the protein, marking it for degradation.

Cystic fibrosis, as too many CTFR proteins are destroyed

What is the general mechanism for transport of proteins in vesicles?

Proteins are segregated to part of the membrane/underlying lumen, membrane buds off to form vesicle as coat forms, vesicle is transported, coat is broken down, vesicle fuses with next membrane

What are vesicle-coating molecules and what are they called?

How do they act?

Small G proteins - Rab proteins

Activated by GTP binding, inactive when GDP bound. Recruited to a membrane, recognises a protein with a certain signal sequence and sheds GDP, GTP binds. At target organelle, a membrane protein hydrolyses the GTP, so rab proteins shed.

What coats can form on vesicles, what do they bind to and where do they direct the vesicle?

COPII, ψψxx, from ER to golgi

COPI, KKxx on KDEL receptor, from golgi to ER

Clathrin, ExxxLL etc. on LDL receptor, from cell membrane to endosomes.