Proteins in Plasma Membranes

Proteins

Proteins make up the second major component of plasma membranes. Integral proteins are, as their name suggests, integrated completely into the membrane structure. In fact, their hydrophobic membrane-spanning regions interact with the hydrophobic region of the the phospholipid bilayer. We refer to some specialized types of integral proteins as integrins. See image below.

The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane. Image Attribution: OpenStax Biology

Single-pass integral membrane proteins usually have a hydrophobic transmembrane segment that consists of 20–25 amino acids. Some span only part of the membrane—associating with a single layer. On the other hand, others stretch from one side of the membrane to the other, and are exposed on either side.

Some complex proteins consist of up to 12 segments of a single protein. These proteins fold extensively and embed themselves in the membrane (see image below). This type of protein has a hydrophilic region or regions, and one or several mildly hydrophobic regions. This arrangement of regions of the protein tends to orient the protein alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid.

Integral membranes proteins may have one or more alpha-helices that span the membrane (examples 1 and 2). Conversely, they may have beta-sheets that span the membrane (example 3). Image Attribution: “Foobar”/Wikimedia Commons

Peripheral Proteins

We can find peripheral proteins on the exterior and interior surfaces of membranes. In fact, they attach either to integral proteins or to phospholipids. Along with integral proteins, peripheral proteins may serve as enzymes. They may also serve as structural attachments for the fibers of the cytoskeleton. Furthermore, they may serve as part of the cell’s recognition sites. We sometimes refer to this as “cell-specific” proteins. The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens.