Transmembrania

Transmembrania, or transmembrane proteins, are proteins that span the phospholipid bilayer of biological membranes, linking the cell's interior with the outside environment. They perform a wide range of essential tasks and come in various architectures. The most common form is one or more alpha-helical transmembrane segments, typically about 20-25 hydrophobic amino acids long, which span the bilayer and provide a hydrophobic core for anchoring. In some membranes, especially outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts, beta-barrel transmembrane proteins also occur.

Topology and orientation are influenced by sequence signals and the positive-inside rule: the distribution of positively charged residues tends to be on the cytoplasmic side, guiding orientation of the protein as it inserts into the membrane. Transmembrane proteins can be single-pass, multi-pass, or form β-barrel pores.

Biogenesis: in eukaryotes, most transmembrane proteins are synthesized on cytosolic ribosomes and targeted to the endoplasmic reticulum. Insertion is mediated by the translocon complex (Sec61) and assisted by chaperones. The correct topology is established during insertion, with luminal/extracellular loops often glycosylated.

Functions: transmembrane proteins function as channels and transporters, receptors, enzymes, adhesion molecules, and anchors. They regulate ion flux, nutrient uptake, signal transduction, cell-cell interactions, and membrane trafficking. Examples include ion channels, G protein-coupled receptors, transporters, and aquaporins.

Overall, transmembrania are central to membrane biology, enabling selective permeability, communication, and coordination of cellular processes.