Passage across a membrane requires a special apparatus

KEY TERMS:

• Protein translocation describes the movement of a protein across a membrane. This occurs across the membranes of organelles in eukaryotes, or across the plasma membrane in bacteria. Each membrane across which proteins are translocated has a channel specialized for the purpose.

KEY CONCEPTS:

• Proteins pass across membranes through specialized protein structures embedded in the membrane.
• Substrate proteins interact directly with the transport apparatus of the ER or mitochondria or chloroplasts, but require carrier proteins to interact with peroxisomes.
• A much larger and complex apparatus is required for transport into the nucleus. 

The process of inserting into or passing through a membrane is called protein translocation. The same dilemma must be solved for every situation in which a protein passes through a membrane. The protein presents a hydrophilic surface, but the membrane is hydrophobic. Like oil and water, the two would prefer not to mix. The solution is to create a special structure in the membrane through which the protein can pass. There are three different types of arrangements for such structures.

The endoplasmic reticulum, mitochondria, and chloroplasts contain proteinaceous structures embedded in their membranes that allow proteins to pass through without contacting the surrounding hydrophobic lipids. Figure 8.2 shows that a substrate protein binds directly to the structure, is transported by it to the other side, and then released.

Peroxisomes also have such structures in their membranes, but the substrate proteins do not bind directly to them. Figure 8.3 shows that instead they bind to carrier proteins in the cytosol, the carrier protein is transported through the channel into the peroxisome, and then the substrate protein is released.

For transport into the nucleus, a much larger and more complex structure is employed. This is the nuclear pore. Figure 8.4 shows that, although the pore provides the environment that allows a substrate to enter (or to leave) the nucleus, it does not actually provide the apparatus that binds to the substrate proteins and moves them through. Included in this apparatus are carrier proteins that bind to the substrates and transport them through the pore to the other side.