Membrane Proteins

Membrane Proteins: Structure and Functions

• Membrane proteins can be categorized into integral and peripheral proteins, with integral proteins being inserted into the lipid bilayer and peripheral proteins being indirectly bound to the membrane by protein-protein interactions.
• Integral proteins can be transmembrane proteins, spanning the lipid bilayer with portions exposed on both sides, and are usually glycoproteins with carbohydrate groups added in the endoplasmic reticulum and Golgi apparatus.
• Transmembrane proteins typically have α helices of 20 to 25 hydrophobic amino acids that are inserted into the membrane during synthesis, and they are transported in membrane vesicles from the endoplasmic reticulum to the Golgi apparatus and then to the plasma membrane.
• Peripheral membrane proteins associate with phospholipid heads or hydrophilic domains of integral proteins by non-covalent interactions and can participate in cell signaling cascades or link the membrane with the cytoskeleton.
• Integral proteins are amphipathic, with their hydrophilic region facing the cytoplasm or extracellular fluid and their hydrophobic domain interacting with the phospholipid tails, enabling them to perform functions such as transferring molecules and signals across the cell membrane.
• Membrane proteins serve various functions including as transporters, enzymes, cell-surface receptors, cell-surface identity markers, cell-to-cell adhesion proteins, and attachments to the cytoskeleton.
• Transport proteins facilitate the movement of molecules into and out of cells and can be channel proteins, such as aquaporins for water passage, or carrier proteins that bind to molecules and shuttle them across the membrane.
• Enzymes associated with the membrane enable different chemical reactions, while receptors receive signals from hormones and transmit them to the cell interior.
• Cell-surface identity markers serve as antigens, and cell-to-cell adhesion proteins act as anchors for cytoskeletal components and receptors for extracellular matrix components, such as integrins for fibronectin and laminin.
• Membrane proteins can also serve as antigens for cell-to-cell recognition and stimulate the production of antibodies, with human surface proteins responsible for cell recognition being called Major Histocompatibility Complex (MHC) or Human Leukocyte Antigens (HLA) markers.
• Membrane proteins can be released from the membrane by relatively gentle extraction procedures, and those proteins are often referred to as peripheral membrane proteins, while transmembrane proteins and proteins held in the bilayer by lipid groups or hydrophobic polypeptide regions cannot be released in these ways.
• Membrane proteins can be anchored to the lipid bilayer by various means, such as amphiphilic α helices, covalently attached lipid chains, or oligosaccharides bound to phosphatidyl inositol (GPI anchor), and some proteins are entirely exposed at the external cell surface and attached to the lipid bilayer only by a covalent linkage to a