Every cell is enclosed by a membrane that separates its interior from the outside world. Studding this surface are membrane proteins, which act as molecular machines embedded within this layer. These proteins are active participants in the life of a cell, performing functions that are fundamental to survival. They allow a cell to interact with its environment, communicate with other cells, and maintain internal stability.
The Architecture of Surface Proteins
Integral proteins are permanently embedded within the membrane and can only be removed by disrupting the structure with detergents. Many are transmembrane proteins that span the entire membrane, with parts exposed to both the cell’s interior and the external environment. These proteins form structures like alpha-helices or beta-barrels to pass through the membrane’s water-repelling core.
Peripheral proteins have a more temporary association with the cell membrane and do not penetrate its core. Instead, they bind to the surface by attaching to the polar heads of lipid molecules or to exposed parts of integral proteins. This allows them to be involved in processes that occur at the membrane’s surface without being fixed in place.
Lipid-anchored proteins are attached to the membrane via a lipid molecule embedded within the bilayer. This covalent attachment provides a stable anchor, holding the protein at the membrane surface to carry out its functions. While firmly attached, they do not enter the membrane’s core in the same way integral proteins do.
Cellular Communication and Recognition
Surface proteins are central to how a cell receives information and communicates. Many function as receptors, shaped to bind with specific signaling molecules like hormones or neurotransmitters. This binding process is highly specific, like a key fitting into a lock. When the correct molecule binds, the receptor changes shape, initiating a cascade of signals inside the cell that leads to a specific response.
Cells also use surface proteins for direct recognition. Glycoproteins, which are proteins with attached carbohydrate chains, are important for this task. These chains project from the cell surface and act as unique identifiers, allowing cells to recognize each other. This process allows cells in a developing organism to assemble into organized tissues and organs.
The immune system relies on this recognition to distinguish between the body’s own cells (“self”) and foreign entities (“non-self”). Immune cells examine the surface proteins of every cell they encounter. If a cell displays the correct “self” identifiers, it is left alone. If a cell is missing these markers or displays foreign ones, the immune system identifies it as a threat and mounts an attack.
Regulating Movement Across the Membrane
The cell membrane is selectively permeable, and surface proteins act as gatekeepers controlling what enters and exits. This regulation is performed by two main types of transport proteins: channel and carrier proteins. Both are highly specific, allowing only one or a few types of molecules to pass through.
Channel proteins form pores through the membrane, allowing specific ions or small molecules to flow through rapidly. These channels can be gated, meaning they open and close in response to specific signals, giving the cell precise control. This rapid transport is important for processes like nerve impulses and muscle contractions.
Carrier proteins bind to a specific substance on one side of the membrane, then change shape to move it to the other side. Some operate through facilitated diffusion, moving substances down their concentration gradient without energy. Others perform active transport, using cellular energy as ATP to pump substances against their concentration gradient, ensuring the cell can accumulate necessary molecules or expel waste.
The Role of Surface Proteins in Medicine
Because they are exposed on the cell surface, membrane proteins are accessible to drugs and are the target for more than half of all modern pharmaceuticals. Malfunctions in these proteins are linked to many diseases. For example, cystic fibrosis is caused by mutations in the gene for the CFTR protein, a channel protein that transports chloride ions. A faulty CFTR protein leads to the thick mucus buildup characteristic of the disease.
Viruses also exploit surface proteins to enter cells. The SARS-CoV-2 virus, for instance, uses its spike protein to bind to the ACE2 receptor on human cells. This binding allows the virus to fuse with the cell membrane and release its genetic material inside, causing infection. Understanding this interaction was instrumental in developing vaccines and treatments.
Drug development often focuses on targeting these proteins. Medications can be designed to either block a receptor to prevent a signal or activate it to produce a desired effect. Beta-blockers, for instance, are drugs that work by blocking receptors for adrenaline, which helps to lower blood pressure and treat heart conditions.