Transmembrane protein

A transmembrane protein is an integral membrane protein that crosses the lipid bilayer of the cell membrane, once (unipass) or several times (multipass). Three domains can be distinguished. Firstly, an area that passes through the membrane, with hydrophobic characteristics (which repels water) to interact with the lipids of the lipid bilayer (cell membrane). In addition, a cytosolic domain and another extracytosolic domain in contact with the interior and exterior of the cell, respectively, of a hydrophilic nature ("they are related to water"). The result is an amphipathic protein.

As an integral protein, it can only be isolated from the bilayer by applying detergents, such as SDS. The main functions are to serve as transport channels for ions or molecules, such as aquaporins that transport water through the membrane; reception of cellular signals, anchorage to the cytoskeleton or to the extracellular matrix, etc.

Types of transmembrane proteins according to their structure/

Different types of transmembrane proteins: 1.- unipaso 2.- multipaste in α-hylice 3.- multipaste in β-lamine

Unistep transmembrane

It passes through the bilayer only once. The N-terminus of the protein is on one side of the membrane, and the C-terminus on the other. Of this type are the PTK-Receptors (with protein tyrosine kinase activity), enzymes of the Golgi apparatus, etc. A well-studied example is glycophorin from the erythrocyte membrane. The domains of these proteins are classified as:

(a) Transmembrane Domain

It has a secondary structure in α-hylice, with a length of about 25 to 30 amino acids. The lateral residues have to be mostly hydrophobic, such as alanine, leucine, isoleucin, etc. to be able to interact with the inside of the bicapa by hydrophobic forces and Van der Waals. It is important to note that the inside of the propeller is occupied, and it does not serve as a channel. At the end of the propeller, there may be residues with load, which interact with the polar head of the bicapa phospholipids.

(b) Non-transmembrane domains

They are usually hydrophilic and globular, of variable length, which are maintained through hydrogen links and electrostatic forces. In the extra-cytosalic domain, disulfuriate links -S-S- and oligosaccharide chains can be found, which does not occur in the cytolic domain.

Multipasse transmembrane

Side view of Acuaporin 1, multi-paste protein in α-hylice, whose interior forms a channel that allows the passage of water. The three-dimensional conformation of these propellers only allows water molecules to pass through the channel,/from one in one.

They cross the lipid bilayer on two or more occasions, usually through several α-helixes, although there are cases of insertions through the β-sheet. There are many examples: receptors associated with trimeric G proteins, ion channels, porins in bacteria, and again in the erythrocyte the Band 3 protein, a multipass transmembrane protein that spans the membrane with 12 α-helices, and with its ends towards the cytosol.

(a) Multipaste in α-hylice

These proteins have as many α-hylices as they sometimes go through the membrane. These propellers are joined by loops in their cytolic and extracytolic domains. The α-hylices can only serve as a medium of anchorage to the bicapa, or can form a channel through which various substances can pass. In the latter case, the propellers have hydrophobic residues that they give out of the channel, to interact with the bicapa; while the residues they give to the internal channel are hydrophilic. In this way, many polar substances that in the absence of proteins could not cross the membrane, now they can. In order to form an ionic channel, a minimum of 5 α-hélices is required.

The α-hale can be moved, slipping on one another, to produce a conformal change. This serves to regulate the opening of channels, transport compounds by permeates and for the transduction of signals

(b) Multipaste in β-lamine

Much less common are cases of multi-paso transmembrane proteins in β-lamine. Examples are given in chloroplasts, mitochondria and bacteria. β-lamine only requires about 10 amino acids to go through the bicapa. There are proteins with a number of β-lamine from 8 to 22.

They usually form channels through which they cross different solutes. It is the case of pore pores, a protein present in the membrane of some bacteria, as well as in mitochondria and chloroplasts, through which substances with a molecular weight less than 800 Da for simple diffusion. Porine crosses the membrane with 16 β-ray anti-parallel, which simulate the duels of a barrel. These foils alternate polar amino acids and apollars, the polars are oriented to the light of the channel, and the apolars contact the hydrophobic part of the lipidic bicapa. This is a difference from the α-hylice. The loops between β-lamine are projected into the channel lumen, which gives them specificity. In addition to porine, other examples are pores also in the form of keg of maltoporine, for maltose, and FepA protein pore, for iron ions, both in bacteria.

However, not all multi-paste proteins in β-lamine act as channels. There are enzymes in β-lamine, such as OMPLA lipasa, and some membrane receptors. In these cases, the polar residues of β-lamine obstruct the channel, impossivizing this function. The only function of the barrel is the anchor to the membrane.

Unlike the α-hylice, the β-chlamine bind each other in a more rigid way, preventing conformal changes, thus limiting their versatility.