ch 18A protein translocation and secretion

What is protein translocation?

The export or secretion of proteins

Explain export of protein translation

Trafficking of proteins from cytosol to CM, surface layers, appendages
periplasm G-
OM G-
Cell wall G+

The extracellular transport of non-proteins.

Explain secretion of protein translation

Trafficking of proteins from cytosol to milieu (environmental medium)

What type of protein is YidC?

Single membrane protein.

What membranes does YidC insert proteins into?

Cytoplasmic membrane of prokaryotes, inner membrane of mitochondria, and thylakoid membrane of chloroplasts.

What role does YidC play in protein folding?

Helps protein folding.

With which segments does YidC interact during protein insertion?

Transmembrane segments of proteins that are inserted via the Sec system.

Can YidC function independently as a membrane protein insertase?

Yes, it can function as an independent membrane protein insertase.

What organisms are the YidC translocation pathway found in?

Bacteria

Euryarchaeota

mitochondria and chloroplasts of eukarya

What organisms are the Sec translocation pathway found in?

Bacteria
Archaea
Chloroplast and cytosol of Eukarya

What organisms are the Tat translocation pathway found in?

Bacteria
Archaea
Chloroplasts of Eukarya

What is the YidC translocation pathway?

A membrane protein insertase

What does the sec and tat system substrates have to facilitate protein translocation?

Signal peptides

What does Tat signal peptide consist of?

Consisits of a twin arginine. folllowed by hydrophobic region, and C region (where signal peptide is cleaved), and mature protein

What does Sec signal peptide consist of?

Positively charge Basic residues, hydrophobic region, c region

What are the type I signal peptides?

Tat and Sec signal peptide

What are the type II signal peptides?

Tat and Sec lipoprotein

What is the difference between type I and II signal peptides

Difference is in the cleavage site.

In type II, a cysteine is lipid modified at the sulfhydral groups and the N terminus of the cleavage gets modified.

Covalent attachment of lipid groups to the mature proteins

What drives the twin-arginine translocation (Tat) system?

Driven by PMF.

Characteristics of Tat (twin arginine translocation) system

Transports folded proteins.

General features of substrates that go through the tat system?

1. membrane proteins that bind cofactors (respiratory proteins, TMAO reductase, photosynthetic transport proteins)

2. cofactorless proteins that need cytosolic factors or that fold too fast for sec transport

3. most proteins of haloarchaea are exported or secreted

The three major examples of proteins going through the tat system?

1. Proteins that bind complex cofactors
2. Proteins that have a specificity for metal insertion.
3. Proteins that form hetero-oligomers.

Explain the model for the Tat translocation cycle in E.coli and plant chloroplasts

1. TatBC binds the signal peptide in an energy-independent step
2.Binding of substrate to TatBC, TatA polymerization.
3. Passenger domain crosses the membrane via TatA.
4. Signal peptide cleavage, and TatA depolymerization.

Does TatBC have a conformational change when bounds to substrate protein?

TatBC does not have a conformational change whether or not its bound to substrate protein.

What forms variable ring structures to accommodate Tat substrates?

TatA

What happens to TatA when it engages in substrate protein?

It can open.

Explain the mechanistic model for changes in TatA oligomerization

after substrate protein engages with TatBC, PMF dependent oligomerization. TatA depolymerizes after substrate removal.