Question

Discuss each of the three mechanisms by which membrane-enclosed organelles import proteins. Construct your response to include where the protein is translated, how it knows where to go, and how it is imported.

Answer 1

Proteins are synthesized by either cytoplasmic ribosomes or by membrane bound ribosomes by attaching on endoplasmic reticulum. All proteins have different different destinations. Proteins move to their target organelles by mainly 3 ways-

By Transport in vesicle form- for example protein transport in Golgi body

By translocation - For example protein transport in mitochondria

By gated channels- for example protein transport in nucleus

Transport in vesicle form-

Such proteins are mostly synthesized by ribosomes which are attached on rough endoplasmic reticulum (RER). Vesicular transport of protein is one of the key ways of membrane transport to golgi apparatus, endoplasmic reticulum as well as to plasma membrane.

Different kind of coat proteins on the vesicles decide that at which cell organelle the protein will be transported,

The vesicles can be coated by different proteins like COP I (for transportation of protein from golgi apparatus to endoplasmic reticulum) , COP II (for transport from rough endoplasmic reticulum to golgi apparatus) and clathrin (for endocytosis, exocytosis and nutrient uptake).

Apart from these coat proteins, there are several other molecules involved in vesicle transport and targeting. Like, Several GTP binding proteins like Ran, Sar 1, ARF. They act as molecular switch.

For example when Sar1 binds with GTP- it triggers start of vesicle assembly.

There are several SNARE proteins, which play an important role in targeting and fusion of vesicles. V SNARE is present on vesicle whereas t SNARE is present on target. They both bind and help in membrane fusion.

Protein transport by translocation-

Many mitochondrial matrix proteins are encoded by cytoplasmic ribosomes. . A leader sequence of 25 to 50 amino acid length is found in matrix proteins destined to be imported inside mitochondria. These proteins are transported inside mitochondria by the help of translocase proteins

Two different types of translocase play a role during this transport

Translocase of outer membrane (TOM)

Translocase of inner membrane (TIM)

Apart from these translocases. Several receptors (TOM 20) along with transmembrane pores are also involved in these imports. For transport inside mitochondria the proteins should be in unfolded state by binding with ATP dependent attachment of chaperones. Chaperones also help in proper folding of proteins once it is imported inside the mitochondria.

Protein transport by gated channels.

.Transport through nuclear pore is a classic example of protein transport through gated channels. Nuclear proteins are synthesized by cytoplasmic ribosomes. Presence of nuclear localization signal in protein makes sure that the synthesized protein will be transported to nucleus,

This transport happens via nuclear pore complexes (NPCs). Several proteins like histones, transcription factors are transported via this method. Proteins of less than 40 K Dalton size does not need any transport mechanism. They can simply sneak through NPC by diffusion. However, in case of larger molecule a key signal called Nuclear localization signal (NLS) must be present in the protein.

NLS are rich in basic amino acids. NPCs are involved in export and import of protein both.

Depending upon NLS type- Importin proteins bind- Ran-GTP transports it inside Nucleus (Import inside Nucleus)

For export of protein outside the nucleus

Different signal is present in protein- called Nuclear export signal (NES). Here Exportins bind to protein and Ran-GDP drives it into cytoplasm.