Protein Transport to the Thylakoids

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.

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.

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.

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.

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.

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.

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.