Proteins in the Cell Membrane

Instructor: Giulietta Spudich

Giulietta has taught college students, graduate students and researchers in scientific topics from genomics to biochemistry. She has a PhD in Molecular and Cell Biology.

Membrane proteins are key to a cell's survival. From receiving messages to transporting important molecules in and out of the cell, proteins have important and dynamic roles. Explore types of proteins in the cell membrane and the roles they play.

What Kind of Proteins are Found in the Cell Membrane?

Just as a castle is surrounded by a wall, a cell has a membrane that separates the inner cellular 'kingdom' from outside its walls. A castle depends on messengers who bring food and news from the outer world. The same is true for a cell. A cell's messengers are proteins that visit the cell membrane from outside the cell (extracellular space). Peripheral proteins are temporarily associated with the cell membrane, and can come and go into intracellular (inside the cell) or extracellular space.

Like people stationed at gatehouses within the castle walls, there are proteins embedded in the cell membrane that work within the membrane itself. These proteins are called integral proteins. Unlike peripheral proteins, integral proteins are tightly associated with the membrane and don't move out of it. Unlike in our gatehouse analogy, where people are stationed at only one place along the castle wall, most integral proteins can move and flow within the membrane.

Let's take a closer look at the membrane itself. A cell membrane is composed of a lipid bilayer (two layers of lipid molecules) that surrounds the cell as a sphere. Integral proteins have a large hydrophobic (water-hating and lipid-loving) component that allows them to exist and function within the lipid membrane. They normally have a smaller hydrophilic part that faces into aqueous, intracellular or extracellular space.

An integral protein in a cell membrane
Figure above: an integral protein in a cell membrane

Largely hydrophobic proteins don't function in aqueous (water-based) solutions, so integral proteins stay in their membrane environment and don't move out into aqueous extracellular or intracellular space. Incidentally, the hydrophobic nature of membrane proteins makes them difficult to study in the laboratory, where many experiments have been designed with water-based environments in mind. However, scientists are facing this hurdle with inventive research techniques.

What Do Membrane Proteins Do?

Let's look at one important function of membrane proteins: receiving news. Receptors are integral proteins in the cell membrane that receive signals from extracellular space so the cell can react. A hormone receptor might get a signal to grow, which sets off a chain of events within the cell called a signalling cascade. In a signalling cascade, the news from the cell membrane is passed from one protein to another within the cell until the right protein (for example, one in the nucleus) gets the message, and the cell can react.

A hormone binds a receptor, causing a signalling cascade
Figure above: a hormone binds a receptor, which causes a signalling cascade

Cell-cell adhesion and structural support are two more key roles membrane proteins play. Cell-cell adhesion, where cells stick to each other, is key when they come together to form tissue or bone. It is also used in the immune response. Cell-adhesion molecules (CAMs) are a large class of membrane proteins involved in this process.

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