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

Mitochondrial Protein Transport

Despite the presence of the mitochondrial genome in the cell, only few mitochondrial proteins are encoded by the mitochondria itself. The vast majority are encoded in the nucleus and consequently synthesized (transcribed) in the cytosol as precursor proteins containing mitochondrial targeting elements that target them to the mitochondria.

Mitochondrial targeting elements exist as single or multiple units scattered along the length of the precursor and vary in terms of sequence, structure and location. 

The most common or "classical" presequence is an N-terminal stretch of 15–55 amino acids which is cleaved upon import.

It is assumed that mitochondrial protein import primarily occurs post-translationally. This necessitates the requirement for cytosolic chaperones to maintain the precursor proteins in their unfolded and import-ready states.

The mitochondria contain four compartments:

• outer mitochondrial membrane (OMM)
• intermembrane space (IMS)
• inner mitochondrial membrane (IMM)
• matrix

How do the cargo precursor proteins overcome the barriers posed by the mitochondrial membranes - especially the outer mitochondrial membrane?

Transport through the Nuclear Pore Complex

As described in the earlier post, the Nuclear Pore Complex (NPC) serves two key purposes:

• to form a barrier of selective permeability within the pore (preventing the passage of nonspecific macromolecules and at the same time allowing the free diffusion of water molecules, sugars and ions) and
• to facilitate transport of selected macromolecules across it

Molecules that go into the nucleus include inner nuclear membrane proteins as well as all the proteins found in nucleoplasm. On the other hand, proteins that go out of the nucleus include those associated with RNA (assembled into ribosomal subunits) and ribonucleoproteins (mRNPs).

Once transported, the NPC must also ensure that the molecules are retained in their respective cytoplasmic and nuclear compartments. This calls for regulation of nuclear transport at multiple stages.

The Nuclear Pore Complex

DNA is enclosed by the nuclear envelope consisting of two concentric membranes - outer and inner nuclear membranes which contain numerous protein complexes called the Nuclear Pore Complexes or NPCs.

The outer nuclear membrane is studded with ribosomes which synthesize proteins that are imported into the perinuclear space (the space between outer and inner nuclear membranes).

The Nuclear Envelope is perforated with Nuclear Pore Complexes

It is estimated that there are around 3000-4000 NPCs in a typical mammalian cell weighing about 125 megadaltons.

Protein Sorting

Protein Mobilization between compartments

Almost all proteins are synthesized on ribosomes in the cytosol but a few find their origin on the ribosomes in the mitochondria or chloroplasts.

These newly synthesized proteins carry out important functions in different organelles where they must be promptly delivered.

But how does the cell know which newly synthesized protein goes to what organelle?