Protein export is a process whereby protein that has been manufactured in a cell is routed to the surface of the cell. Export of proteins occurs in all microorganisms , but has been particularly well-studied in certain species of bacteria and yeast .
The ability of a cell to export protein is crucial to the survival or pathogenicity of the cell. Bacteria that have protein appendages for movement (e.g., flagella) and attachment (e.g., pili), and protective protein surface coatings (e.g., S layers) depend on the efficient export of the proteins. Exotoxins that are ultimately excreted by some bacteria need to get across the cell wall before being released from the bacterium.
Defects in protein export can produce or contribute to a number of maladies in eukaryotic cells including human cells (e.g., cystic fibrosis, diabetes, osteopororsis).
A general feature of protein export is the manufacture of a protein destined for secretion in a slightly longer form than the exported form of the protein. The additional stretch is known as the signal sequence, and its role in protein export forms the so-called signal hypothesis . Gunter Blobel garnered the 1999 Nobel Prize in Physiology or Medicine for his pioneering efforts in this area.
The "pre-protein" contains sequences of amino acids that give the precursor stretch of protein blocks that are hydrophilic (water-loving) and hydrophobic (water-hating). This allows a portion of the precursor region to spontaneously bury itself in the membrane layer that surrounds the interior of a bacterium, or the membrane of the endoplasmic reticulum of cells such as yeast. The hydrophilic sequences that flank the hydrophobic region associate with either side of the membrane. Thus, the precursor region is a membrane anchor.
Anchoring of the protein to the membrane is assisted by the action of two proteins. One of these proteins (designated SecB) associates with the precursor sequence of the newly made protein. The SecB protein then recognizes and binds to a protein called SecA that is embedded in the membrane. The SecB-SecA complex acts to guide the precursor region into position at the membrane. As the remainder of the protein is made, it is pushed out of the opposite side of the membrane. An enzyme associated with the outer surface of the membrane can snip off the precursor.
This process is sufficient for protein export in Gramnegative bacteria that have the single membrane. However, in Gram-negative bacteria the protein must be transported across the periplasm and the outer membrane before being truly exported. Furthermore, yeast cells require additional mechanisms to route the protein from the Golgi apparatus to the exterior of the cell.
Proteins destined for export in Gram-negative bacteria are also synthesized as a precursor. The precursor functions at the outer membrane. Thus, the precursor must cross the inner membrane intact. This occurs because of an association that forms between a newly made precursor protein and a complex of several proteins. The protein complex is referred to as translocase. The translocase allows the precursor protein, with the hydrophobic region, to be completely transported across the inner membrane.
Studies using Escherichia coli and Haemophilus influenzae demonstrated the molecular nature of the translocase effect. The SecB protein is associated with the precursor region as a channel running alongside the precursor. The channel has a hydrophilic and a hydrophobic side. The latter is oriented outward so that it partitions into the hydrophobic interior of the bacterial inner membrane. The inner surface of SecB that is in intimate contact with the precursor region is hydrophilic. Thus, the precursor moves through the inner membrane in a watery channel.
As the precursor emerges into the periplasm, another protein present in the periplasm associates with the precursor region. This association also protects the precursor and allows the precursor to reach the inner surface of the outer membrane. Once there, the periplasmic protein is released, and the precursor sequence spontaneously inserts into the outer membrane.
Protein export has become an important target of strategies designed to thwart microorganism infections. By blocking the ability of certain proteins to be exported, the ability of bacteria to establish an infection can be hindered.
Conversely, the engineering of proteins to encourage their export can allow for the easier purification of commercially and clinically important proteins. For example, the engineering of human insulin in Escherichia coli relies on the export of the insulin protein. Once free of the bacteria, the insulin can be recovered in pure form much more easily and economically than if the protein needed to be extracted from the bacteria.
See also Bacteria and bacterial infection; Bacterial membranes and cell wall; Bacterial movement; Bacterial surface layers; Bacterial ultrastructure; Cell membrane transport; Enterotoxin and exotoxin; Molecular biology and molecular genetics; Prokaryotic membrane transport; Proteins and enzymes