Question

How are proteins sorted into their appropriate cell compartments in eukaryotes?

Answer 1

In eukaryotic cells, internal membranes divide the cell up into sub-cellular compartments. These compartments, sometimes called membrane-bounded organelles include the nucleus, the endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, etc. The presence of these enclosed compartments allow cells to organize the thousands of chemical reactions they carry out and to confine certain metabolic processes to particular organelles. While this is advantageous to the cell, it poses a problem. All proteins, no matter which cellular compartment they function in, are synthesized by ribosomes in the cytosol. In order to get proteins into the different compartments they belong in, the cell must
a. have ways of identifying which organelle a protein is destined for and
b. have ways of delivering the protein into the appropriate organelle.
This means the proteins have to cross one or more membranes. This is no trivial issue given the size of even small proteins. We will now consider the strategies employed by cells to achieve the correct "addressing" and "delivery" of proteins to the various sub-cellular compartments.

There are many membrane enclosed compartments in eukaryotic cells. Some of the prominent ones include the nucleus, the ER, the Golgi Apparatus, mitochondria, chloroplasts, peroxisomes and lysosomes.

• What is protein sorting :-

The vast majority of proteins are made by ribosomes in the cytoplasm of the cell (the exceptions are a handful of proteins made within mitochondria and chloroplasts). Each of the thousands of proteins made in the cytoplasm must be delivered to the cellular compartment in which it functions. This is roughly analogous to the thousands of letters at a post office that must be sorted and delivered to their recipients in different locations. The process by which proteins are identified as belonging to a particular compartment and then correctly delivered to that destination is known as protein sorting.

• How does a cell know where a particular protein should be sent :-

Proteins have "address labels" or sorting signals that indicate which cellular compartment they are destined for. The sorting signals are part of the amino acid sequence of the proteins. Characteristic sorting signals are found on proteins that are sent to the nucleus, the mitochondria, etc.

• What do sorting signals look like :-

Most sorting signals (called signal sequences) are short stretches of amino acid sequence. They are literally the "address label" that indicates where the protein should be sent. Different cellular compartments have different "address labels".

• How do proteins cross membrane barriers to get into the different compartments of the cell :-

The mechanism by which proteins cross membranes to get into different organelles varies. For example, proteins going into the nucleus are transported through pores in the nuclear membranes with the help of nuclear transporter proteins. Nuclear proteins are transported in their folded state. However, proteins going into the ER enter the lumen (interior) of the ER through protein translocators in the ER membrane. In this case, the proteins must enter in an unfolded form, since the dimensions of the translocator will not allow a folded protein to pass through. A third mechanism encloses proteins inside a membrane vesicle, which travels to the target organelle and fuses with the membrane of the target.

• How do proteins get into the nucleus :-

To get into the nucleus, proteins must, first of all, have a signal sequence called a Nuclear Localization Signal or NLS. The NLS typically is a stretch of positively charged amino acids like lysine and arginine. This serves as the label that marks the protein for delivery to the nucleus. In addition, nuclear proteins must cross the nuclear envelope to get inside the nucleus.

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