Protein Transport in Chloroplasts

Like mitochondria, chloroplasts must correctly localize proteins encoded in the nucleus and subsequently synthesized in the cytosol. Protein transport in chloroplasts is similar to that in mitochondria with a few exceptions:

Features	Mitochodria	Chroloplasts
Process occurs	post-translationally	post-translationally
Translocation complexes	separate in each membrane	separate in each membrane
Requires energy?	Yes	Yes
Requires signal sequences?	N-terminal, removed after import	N-terminal, removed after import
Requires chaperones?	Yes	Yes
Requires gradient across membrane?	Electrochemical (H+) in inner membrane	Electrochemical (H+) in thylakoid membrane
Requires ATP/GTP hydrolysis?	No	Yes

A major difference between protein transport in mitochondria and chloroplasts is that in the case of the latter, proteins are first transported into the chloroplast matrix referred to as the stroma. The precursors of these proteins possess cleavable N-terminal sequences called "transit peptides" that act as a chloroplast signal sequences and target them to chloroplasts.

Protein transport in chloroplasts. Albert et al. 4th Edition

Immediately after the chloroplast signal sequence is a hydrophobic thylakoid signal sequence that is unmasked after cleavage of the former by stromal processing peptidase (SPP) similar to the mitochondrial processing peptidase (MPP) once the precursor protein enters the stroma.

Once the thylakoid signal sequence is exposed, transport of the precursor to the thylakoid membrane is initiated. This type of transport can occur by any one or more of the following four pathways:

1. Sec pathway - uses chloroplast Sec (cpSec)
2. SRP-like pathway - uses chloroplast Signal Recognition Particle (cpSRP)
3. pH gradient/cpTat pathway - has a double arginine (RR) motif next to the hydrophobic domain; uses proton gradient to drive the transport reaction.
4. Spontaneous insertion pathway - does not require any protein translocators 

In the past decade, much research has been carried out to understand protein trafficking into the chloroplast. The TOC (Translocon at the outer chloroplast envelope) and TIC (Translocon at the inner chloroplast envelope) are newly discovered translocases that import most chloroplast proteins and deliver them to the stroma.

The TOC-TIC system functions as the general import system and is responsible for import of most of the chloroplast proteins. Although TOC and TIC are separate translocases they act in concert at place where they come in contact such as envelope membrane contact sites. Both transport proteins in an unfolded conformation and both possess the following essential components:

• Receptors - to receive the precursor proteins at the chloroplast surface
• Transmembrane channels - to allow protein translocation
• Motor proteins - to drive precursor translocation through the channels