Membrane Proteins: Functions, Types & Structure

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  • 0:01 Review of Membrane Biology
  • 1:35 Membrane Protein Functions
  • 2:34 Membrane Protein Types…
  • 4:34 Lesson Summary
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Lesson Transcript
Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

All membrane proteins have one thing in common, which is that they are all associated with a membrane. As a group, however, they are structurally and functionally highly diverse. In this lesson, we will explore the different types of membrane proteins, how they function, and their structure.

Review of Membrane Biology

In all cellular organisms, the plasma membrane is the outermost layer of a cell. It serves to separate the cell from the external environment. In prokaryotes, it is the only membrane. Eukaryotes have membrane-bound organelles. In all cases of membranes, they are made of the same basic phospholipid bilayer.

Membrane proteins are anchored in the phospholipid bilayer. The bipolar nature of this molecule is the key to how membranes work. Each phospholipid molecule has a hydrophilic (or water-loving) head region (the phospho portion) and a hydrophobic (or water-fearing) tail region (the lipid portion). Hydrophilic regions want to be in an aqueous (or watery) environment. Hydrophobic regions want to be in a non-aqueous environment. In this figure, you can see the arrangement of the phospholipid bilayer and examples of membrane proteins.

A phospholipid bilayer has multiple types of membrane proteins
Phospholipid bilayer diagram

So, what gives a membrane its shape? It is the way phospholipids will arrange themselves. The hydrophilic heads love water, so they want to face the outside or inside of the cell. The hydrophobic tails want to be away from the water, so they will interact with each other. So, the tails snuggle into each other and away from aqueous regions. The heads are on the surface in contact with the aqueous environment.

Membrane Protein Functions

As their name suggests, membrane proteins are anchored in membranes. However, each of them has a specific function, and that function often requires them to extend beyond the membrane. Some membrane proteins sit on the surface. Others are anchored in the membrane and have pieces (called domains) on one or both sides of it.

Proteins with extracellular domains (which means they're outside the cell) are usually involved with cell-to-cell communications or interactions. Proteins that reside mainly within a membrane usually form channels or pores to allow molecules to cross the membrane. Proteins with cytosolic domains (which means they're inside the cell) have the widest range of function. For example, they can be anchors for cytoskeletal (cell skeleton) proteins or involved with intracellular signaling.

Membrane Protein Types and Structures

The structure of membrane proteins often determines their function. This, in turn, is the way we classify the type of membrane protein. In general, integral proteins are embedded within the hydrophobic layer of the membranes. Peripheral proteins are not embedded within the hydrophobic region of the membrane. Instead, they are associated with the hydrophilic heads or with an integral membrane protein.

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