Question 1

What are the three main pathways for protein transport to thylakoids?

Accepted Answer

Thylakoid proteins use three distinct sorting pathways. The cpSec pathway translocates unfolded precursors using ATP-dependent motor proteins through the cpSecY channel. The cpTat pathway translocates folded proteins across Tat pores using the proton gradient's electrochemical potential. The cpSRP pathway transports unfolded proteins via GTP hydrolysis through a cpSRP translocase, facilitating insertion into thylakoid membranes.

Question 2

How does the cpTat pathway recognize and transport proteins to the thylakoid lumen?

Accepted Answer

The cpTat pathway recognizes twin-arginine (RR) motifs in thylakoid signal sequences. These residues induce oligomerization of Tat proteins to form pore-like complexes. Folded proteins are then translocated across these Tat pores into the thylakoid lumen using energy from the electrochemical potential of the proton gradient across the thylakoid membrane.

Question 3

What role do stromal processing peptidases play in thylakoid protein targeting?

Accepted Answer

Stromal processing peptidases remove transit signals from precursor proteins after they reach the stroma via the TOC/TIC import pathway. This removal exposes thylakoid signal sequences, allowing processed precursors to be recognized by specific sorting pathways. Additionally, thylakoid processing peptidases cleave signal-like peptides from inserted membrane proteins to form mature transmembrane proteins.

Question 4

How does the cpSRP pathway insert chlorophyll-binding proteins into thylakoid membranes?

Accepted Answer

The cpSRP pathway transports chlorophyll-binding proteins to the thylakoid membrane using energy from GTP hydrolysis. cpSRP proteins and FtsY receptors bind processed precursors and stimulate GTP hydrolysis. This energy facilitates precursor entry through the Alb3 translocase and their subsequent insertion into the thylakoid membranes.

Question 5

What distinguishes the cpSec pathway from other thylakoid protein transport mechanisms?

Accepted Answer

The cpSec pathway uniquely translocates unfolded proteins across the thylakoid membrane using an ATP-dependent cpSecA motor protein that pulls peptides through the cpSecY channel. Unlike the cpTat pathway, which transports folded proteins, or the cpSRP pathway, which uses GTP hydrolysis, the cpSec pathway requires ATP energy and unfolded precursor substrates.

Question 6

How do some thylakoid membrane proteins insert without energy input or targeting factors?

Accepted Answer

Some small thylakoid membrane proteins employ Oxa-like integrases for spontaneous insertion without energy input or targeting factors. These proteins form soluble stromal intermediates with short hydrophobic signal-like peptides that allow them to fold. Once inserted into the membrane, thylakoid processing peptidases cleave the signal-like peptides to generate mature transmembrane proteins.

Question 7

Why do cpTat peptidases specifically cleave only cpTat precursors and not cpSec precursors?

Accepted Answer

cpTat peptidases possess a basic amino acid in their C-terminal domain that distinguishes cpTat precursors from cpSec precursors. This structural feature allows cpTat peptidases to selectively recognize and cleave only cpTat precursors carrying the twin-arginine motif, preventing inappropriate cleavage of cpSec pathway substrates.

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