Biological membranes encircle eukaryotic cells and their components, or organelles. Membranes allow selective permeability to the cell or organelle, and prevent spontaneous diffusion of water and other molecules. Thanks to the surrounding membrane, idyllic conditions, such as pH and ionic strength, can be maintained within the cell, despite exterior conditions that may be very different. Biological membranes are two-layered structures known as phospholipid bi-layers. Each molecule within the bi-layer has both polar and non-polar characteristics. Polar molecules are hydrophilic, meaning water loving, while non-polar molecules are hydrophobic, and cluster together in order to keep from reacting with water. The polar heads of the bi-layer line the exterior and cytosolic surfaces of the cell, while the non-polar tails huddle together within the interior core. Proteins that interact with membranes play either a direct or indirect role in a vast number of biological processes. Thousands of proteins have been identified. At least half are associated with biological membranes. Some membrane proteins regulate the passage of water and other molecules into and out of the cell, some provide structural support, and some are receptors for recognition molecules. There are three types of membrane proteins, integral, lipid-anchored, and peripheral.
Integral membrane proteins, also known as trans-membrane proteins, contain hydrophobic domains embedded within the plasma membrane. Some integral membrane proteins pass through the membrane only once, while others cross over as many as seven or more times. Hydrophilic domains on either the cytosolic or the extracellular surface of the membrane are able to interact with an aqueous environment, while the membrane-embedded domains of the protein remain packed within the hydrophobic core of the bi-layer.
Many different integral membrane proteins function in service to the cell. Some operate as transport proteins, moving molecules into or out of a cell. Some transport proteins allow facilitated diffusion, allowing molecules to travel down their concentration gradient. Others provide active transport across a membrane against a concentration gradient, a process requiring an input of energy. Some integral membrane proteins form ion channels to allow the flow of charged atoms down their concentration gradient, in order to maintain a voltage gradient across the membrane. Still others are receptors on the surface of the membrane to which a signaling molecule, such as a hormone, for example, binds, initiating a response within the cell. Some integral proteins provide structural support, and some are adhesion molecules, which interact with molecules on the surface of other cells in order to ensure tight associations between cells.
Lipid-anchored proteins have a hydrophobic carbon chain buried within the core of the bi-layer, anchoring the protein to the membrane, and a hydrophilic domain at the surface. One important family of lipid-anchored proteins are the G proteins, which act as molecular “switches” and serve to regulate certain cellular processes. Another group of lipid-anchored proteins, the kinases, transfer phosphate groups from high-energy molecules such as ATP to target molecules, a major step in thousands of metabolic processes.
Peripheral membrane proteins do not penetrate the hydrophobic core, and some only associate temporarily with the membrane. These proteins may have indirect interactions with integral membrane proteins, or with the polar head group, and are localized to either the cytosolic or the extracellular surface of the membrane. Cytoskeletal proteins, for example, provide support to the membrane, but do not form a bond within the membrane. Some peripheral membrane proteins are adhesion molecules, aiding in the formation of “tight junctions” between cells. Cytochrome C is one example of a well-studied peripheral membrane protein. Part of the electron transport chain within the mitochondria, Cytochrome C associates with the integral membrane proteins of the electron transport chain, but does not penetrate the inner membrane of the mitochondria (see mitochondria function and structure).
Membrane proteins have functions critical to the survival of the cell. Without them nutrients and metabolites could not enter the cell, and waste products could not pass out of the cell, voltage gradients could not be maintained, and thousands of metabolic processes could not go forward. Biological membranes provide superlative protection to cells and their organelles, but without their associated proteins, membranes would be downright useless.