Classification of Membrane Proteins
Based on their relationship to the lipid bilayer, membrane proteins are divided into two main groups:
- Integral (transmembrane) proteins. Their polypeptide chain is partially or fully embedded within the lipid layer. Their structure is zonal:
- The hydrophobic zone interacts with the hydrocarbon chains of fatty acids and is enriched in nonpolar amino acids.
- The hydrophilic zone faces the polar lipid headgroups and contains predominantly hydrophilic amino acids.
- Peripheral proteins. These attach to the membrane in various ways:
- Binding to pre-existing integral proteins.
- Interacting directly with polar lipid headgroups.
- Anchoring via a short hydrophobic terminal domain.
- Anchoring via a covalently attached acyl residue.
Amphipathicity and Protein Modifications
Proteins contacting the hydrophobic core of the bilayer must be amphipathic, possessing a distinct nonpolar domain. This is achieved through amino acid composition (clustering of nonpolar amino acids) and acylation (covalent attachment of fatty acid residues).
Within the cell, proteins undergo various modifications, each serving a specific purpose:
- Acylation. Fatty acids act as an anchor, securing the protein in the membrane while allowing lateral diffusion (movement along the plane of the layer).
- Glycosylation. Occurs on the extracellular 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-tuning of protein function.
Main Functions of Membrane Proteins
In addition to maintaining transmembrane asymmetry (compositional differences between the outer and inner leaflets), proteins perform a wide spectrum of tasks:
- Transport: Ensure strictly selective transport of substances into the cell and elimination of metabolites.
- Receptor: Participate in receiving and transmitting hormonal signals into the cell.
- Vesicular transport: Essential for forming coated pits, which mediate endocytosis and exocytosis.
- Immunological: Play a key role in recognition and immune responses.
- Enzymatic: Act as catalysts for various biochemical reactions.
- Structural and adhesive: Organize intercellular junctions, which are crucial for the formation of tissues and organs.