Transmembrani

Transmembrani, or transmembrane proteins, are molecules that span biological membranes, crossing the hydrophobic core of the lipid bilayer. They are typically integral membrane proteins whose hydrophobic segments anchor them within the membrane. Transmembrane proteins mediate communication between the cell and its environment, regulate transport of ions and molecules, and organize membrane domains. They can extend across the membrane once (single-pass) or multiple times (multi-pass).

Structure and topology: Most transmembrane proteins adopt alpha-helical or beta-barrel architectures. Alpha-helical proteins are prevalent in eukaryotic plasma membranes, whereas beta-barrels occur mainly in the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts. The orientation and number of spans are determined by amino acid composition and signal sequences; the positive-inside rule biases cytosolic sides to be positively charged, guiding insertion and topology.

Functions: They serve as channels and transporters, enabling facilitated diffusion and active transport; receptors for signals; enzymes; and mediators of cell adhesion and immune recognition. Some proteins function as enzymes within membranes, generating signaling lipids or converting substrates while tethered to the bilayer.

Examples and importance: Notable examples include G protein-coupled receptors, ligand-gated and voltage-gated ion channels, and transporters such as solute carriers and pumps like the Na+/K+ ATPase. Transmembrane proteins are essential for cellular homeostasis, nervous signaling, metabolism, and development. Mutations affecting transmembrane regions can cause diseases, and many drugs target transmembrane proteins to alter signaling or transport.

Research notes: Prediction of transmembrane spans uses hydrophobicity analyses; structures are elucidated by X-ray crystallography, cryo-electron microscopy, and related methods. Comparative genomics reveals conserved motifs that guide folding and function.