But the synthesis of proteins is only the beginning. The newly minted proteins are transported around the cell, team up with other proteins, and do amazing tasks. Some of the proteins which are stationed nearby the cell membrane have an oily tail attached (by another protein) which inserts into the membrane and so anchors the protein. Such an anchor is also used to transport certain membrane proteins.
The oily tail anchors the proteins to an internal membrane. Small membrane bubbles, called vesicles, are pinched off from the internal membrane, taking with them proteins which are anchored. The vesicles transport the protein to the cell outer membrane.
Eventually these proteins disassociate from the cell membrane, have their oily tail detached (by yet another protein), and make their way back to the internal membrane where they have a new tail attached and are transported back to the cell outer membrane for more duty. Here is how a recent research paper described the process:
Palmitoylation is detectable only on the Golgi, whereas depalmitoylation occurs everywhere in the cell. The reactions are not stereoselective and lack any primary consensus sequence, demonstrating that substrate specificity is not essential for de-/repalmitoylation. Both palmitate attachment and removal require seconds to accomplish. This reaction topography and rapid kinetics allows the continuous redirection of mislocalized proteins via the post-Golgi sorting apparatus. Unidirectional secretion ensures the maintenance of a proper steady-state protein distribution between the Golgi and the plasma membrane, which are continuous with endosomes. This generic spatially organizing system differs from conventional receptor-mediated targeting mechanisms and efficiently counteracts entropy-driven redistribution of palmitoylated peripheral membrane proteins over all membranes.
Incredibly evolutionists describe this process as simple:
Cells thus use a simple principle to transport Ras and other palmitoylated proteins to their destination: a localised distribution centre (Golgi), directed transport to the target destination as well as universal removal of target marks (depalmitoylierung) and subsequent reintegration into the transport cycle.
One reason why this transport cycle is clever is that the oily tails are only attached to (and detached from) cysteine amino acids located on the surface of these membrane proteins. So having a cysteine available on the surface could be a way to “tag” these proteins that should be sent to the membrane:
But how does the cell know which proteins need to be addressed to the cell membrane whilst they are in the Golgi apparatus? According to the scientists, any protein can obtain a lipid anchor if it has the amino acid called cysteine readily accessible on its surface. It would then be transported automatically to the cell membrane. Such transportation therefore does not require any receptors which specifically bind to the protein at the cellular site where it is supposed to go.
This is a fascinating example of how complex processes can be controlled with simple physical and chemical rules. At first glances, it would appear to be enormously challenging to identify the proteins that need to be transported to a certain location, to spot any that have been transported to the wrong place and to stop them radiating off from their ultimate destination. Yet the cell manages this in a really simple way without any additional receptors or regulatory mechanisms.
Yes that is clever. But it also requires extremely complex and unlikely amino acid encoding. This simple rule would require that a cysteine end up on the surface of these proteins after they fold. And likewise, the rule would require that there be no cysteines on the surface of other proteins, which are not supposed to be so transported to the membrane, after they fold.
In other words, the identification problem has just been pushed back to an earlier stage in the process.