Classification of Membrane Proteins
Based on their position relative to the lipid bilayer, membrane proteins are divided into two main groups:
- Integral (transmembrane) proteins. Their polypeptide chain is partially or fully embedded in the lipid layer. The structure of such proteins is zonal:
- Hydrophobic zone interacts with the hydrocarbon chains of fatty acids. It is enriched with nonpolar amino acids.
- Hydrophilic zone is located in the region of the polar lipid "heads" and contains predominantly hydrophilic amino acids.
- Peripheral (surface) proteins. They attach to the membrane in various ways:
- Bind to already embedded integral proteins.
- Attach directly to the polar lipid "heads".
- "Anchor" into the membrane via a short hydrophobic terminal domain.
- "Anchor" via a covalently linked acyl residue.
Amphiphilicity and protein modifications
Proteins contacting the hydrophobic part of the bilayer must be amphiphilic, meaning they have a pronounced nonpolar domain. This property is achieved in two ways: through a specific amino acid composition (clustering of nonpolar amino acids) and via acylation—the covalent attachment of fatty acid residues.
Within the cell, proteins undergo various modifications, each serving a specific purpose:
- Attachment of acyl residues. Fatty acids act as an "anchor", securely holding the protein in the membrane while still allowing it lateral diffusion (movement along the layer).
- Glycosylation. Occurs on the outer surface of the membrane. Carbohydrate components protect integral proteins from the destructive action of extracellular proteases.
- Phosphorylation. This post-translational modification is essential for the fine regulation of protein functions.
Main functions of membrane proteins
In addition to maintaining transmembrane asymmetry (compositional differences between the outer and inner leaflets), proteins perform a vast range of tasks:
- Transport. Provide strictly selective transport of substances into the cell and excretion of metabolites from it.
- Receptor. Participate in receiving and transmitting hormonal signals into the cell.
- Involvement in vesicular transport. Essential for the formation of "coated pits", through which endocytosis and exocytosis occur.
- Immunological. Play a key role in the recognition and execution of immune responses.
- Enzymatic. Act as catalysts for various biochemical transformations.
- Structural and adhesive. Organize intercellular contacts, which is absolutely essential for the formation of fully functional tissues and organs.
Briefly: answers to topical questions
Which enzymes catalyze the phosphorylation of membrane proteins?
Phosphorylation of membrane proteins in the cited sources is directly described for the following enzymes/receptor systems:
- Protein kinase A (PKA) — catalyzes the transfer of a phosphate group from ATP to the regulatory protein/domain R of the chloride channel complex: R + ATP → R-P + ADP.
- Tyrosine kinase receptors — undergo autophosphorylation upon hormone binding and trigger a protein phosphorylation cascade.
Which specific proteins form coated pits during vesicular transport?
Coated pits in receptor-mediated endocytosis are formed with the participation of coat proteins:
- Adaptin — acting as a linking element.
- Clathrin — forming the vesicle scaffold.
After a substance binds to a receptor on the membrane surface, invagination occurs with the formation of a coated pit involving adaptin and clathrin, followed by the formation of a vesicle surrounded by a clathrin coat.
What is the mechanism of lateral diffusion of integral proteins in the membrane?
Lateral diffusion is the ability of membrane components to move within the plane of the membrane without leaving their layer. This property is associated with the fluid-mosaic structure of the membrane.
For integral proteins, the sources explicitly cite the example: G protein-coupled adrenaline receptors are integral membrane proteins with 7 transmembrane domains capable of lateral diffusion within the membrane bilayer.
Integral proteins have a portion of their polypeptide chain immersed in the lipid layer; their hydrophobic regions interact with the hydrocarbon chains of fatty acids and contain nonpolar amino acids.
To help remember the functions of membrane proteins, use the mnemonic: "Transport Receptors Proceed Enzymatically and Structurally" (Transport, Receptor, Immunological, Enzymatic, Structural).
Class and exam questions
What does the term "transmembrane asymmetry" mean?
This is a structural feature of the membrane where the outer and inner lipid layers differ from each other in the composition of lipids and embedded proteins.
How can peripheral proteins be retained on the membrane?
They can bind to integral proteins, attach to polar lipid heads, or "anchor" using a short hydrophobic domain or a covalently attached fatty acid residue (acylation).
What is the purpose of protein glycosylation on the outer surface?
The attachment of carbohydrate components protects integral membrane proteins from damage by protease enzymes located in the extracellular space.
What else to study on this topic
- Mechanisms of coated pit formation during vesicular transport
- The process of post-translational phosphorylation for activity regulation
- Differences in the amino acid composition of hydrophobic and hydrophilic zones of integral proteins
- The role of lateral diffusion of acylated proteins
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