What is Membrane Transporter?

Cellular life strongly depends on the membrane ability to precisely control exchange of solutes between the internal and external (environmental) compartments. This barrier regulates which types of solutes can enter and leave the cell.

Transporters are integral membrane proteins with central roles in the efficient movement of molecules across biological membranes. Many transporters exist as oligomers in the membrane. Depending on the individual transport protein, oligomerization can have roles in membrane trafficking, function, regulation and turnover.

Classification of Membrane Transporter:

Most of molecules enter or leave cells mainly via membrane transport proteins, which play important roles in several cellular functions, including cell metabolism, ion homeostasis, signal transduction, binding with small molecules in extracellular space, the recognition process in the immune system, energy transduction, osmoregulation, and physiological and developmental processes. There are three major types of transport proteins, ATP-powered pumps, channel proteins and transporters.

Transporters, a third class of membrane transport proteins, move a wide variety of ions and molecules across cell membranes. Membrane transporters either enhance or restrict drug distribution to the target organs. Depending on their main function, these membrane transporters are divided into two categories: the efflux (export) and the influx (uptake) transporters.

Figure 1 Membrane transporter/Ion channel

What is Ion Channel?

Ion channels are membrane proteins, found in virtually all cells, that are of crucial physiological importance. Ion channels are protein molecules that span across the cell membrane allowing the passage of ions from one side of the membrane to the other. Ion channels are typically assembled from many subunits that form the pore-lining structure. The number of subunits that form ion channels varies from subfamily to subfamily. Ion channels are integral membrane proteins that span through the cell membranes to form a pore that can be penetrated by selected ions at a rate of up to 100 million per second. They are found in virtually all organisms from viruses and bacteria to plants and animals, and in all cell types of the human body. The ion channels consist of pore-forming and, sometimes, accessory subunits. Most ion channels have specialized gate mechanisms, capable of making transitions between conducting and nonconducting states.

Figure 2 Summary of channel-transporter complex formation and functional crosstalk

Classification of Ion Channels in the Body:

--Voltage-Gated Ion Channels

Voltage-gated channels respond to perturbations in cell membrane potential, and are highly selective for a specific ion, i.e., Na⁺, K⁺, Ca²⁺, and Cl^-. They are further subdivided into families based on the major permeant ion.

--Ligand-Gated Ion Channels (LGICs)

LGICs are targets for many drugs, such as anesthetics, antipsychotics, and antidepressants. They are named according to the ligand to which they respond and are divided into three families based upon molecular biology and protein structure criteria.

--"Cys-Loop" LGIC

They constitute the largest class of LGICs and include nicotinic acetylcholine receptors (nAChR), γ-aminobutyric acid (GABA) receptors, 5-hydroxytryptamine-3 (5HT3) receptors, and glycine receptors. The nAChR and 5-HT3R are excitatory receptors, while the GABA receptors and glycine receptors are inhibitory in nature.

--Ionotropic Glutamate Receptors

They are activated by the neurotransmitter glutamate and mediate most fast excitatory transmission in the CNS. They are subdivided into α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors, kainate receptors and N-methyl-D-aspartate (NMDA) receptors.

--P2X Receptors

They are the most recently discovered membrane ion channels. They are preferably permeable to Na_⁺, K_⁺ and Ca_²⁺ and are activated by ATP. P2X receptors are widely expressed in many tissues and are shown to play key roles in various physiological processes, such as nerve transmission, pain sensation, and various inflammatory responses.

--Mechano-Sensitive Ion Channels

Ion channels responding to changes in mechanical forces on the cell membrane are termed mechano-sensitive ion channels. These channels are involved in detection and transduction of external mechanical forces into electrical and/or chemical intracellular signals.

Characteristics and Difference Between Ion Channel and Transporter:

All living cells require membrane proteins that act as conduits for the regulated transport of ions, solutes and other small molecules across the cell membrane. Ion channels provide a pore that permits often rapid, highly selective and tightly regulated movement of ions down their electrochemical gradient.

Ion channel and transporter are two types of transmembrane proteins that control the movement of ions across the cell membrane. Both ion channel and transporter aid the selectively permeable nature of the cell membrane by allowing only the selected molecules to pass through the cell membrane. The main difference between ion channel and transporter is that the movement of ions occurs through a concentration or electrochemical gradient in ion channels whereas the movement of ions occurs against the concentration gradient in transporters. Transporters are also called ion pumps. Ion channels are fast transporters while transporters make slow translocations.

References:

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