transmembranen

Transmembranen, or transmembrane structures, are components that span biological membranes. In biology, the term most often refers to transmembrane proteins that traverse the lipid bilayer, though the concept can also describe other spanning features within proteins. These structures enable communication, transport, and signaling across membranes and are essential to cellular function.

Most transmembrane proteins in eukaryotes are α-helical, containing several hydrophobic segments that cross the bilayer. Each segment is usually 20–25 amino acids long and adopts an α-helix that anchors the protein in the membrane. The arrangement of these segments determines the protein’s topology, with the positive-inside rule biasing the orientation of cytosolic versus extracellular termini.

In other membranes, notably the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts, transmembrane proteins can form β-barrels composed of antiparallel β-strands that create pores. These β-barrel proteins differ in structure and assembly from α-helical ones and are common in organellar and bacterial membranes.

Insertion and folding are assisted by cellular machinery. Most transmembrane proteins are translated on ribosomes associated with the endoplasmic reticulum and are co-translationally inserted via the Sec61 translocon, aided by chaperones. Lipid composition and membrane thickness also influence folding and stability.

Functions include transport of ions and small molecules, signaling and receptor activity, enzymatic catalysis, and cell adhesion. Notable examples are aquaporins (water channels), GPCRs (seven-transmembrane receptors), voltage-gated ion channels, and porins. Methods to study transmembranes include X-ray crystallography, cryo-electron microscopy, and topological analyses such as hydropathy profiling.