Fig. 1.7 Plasma membrane organization and asymmetry. (A) The plasma membrane is a bilayer of phospholipids, cholesterol, and associated proteins. The phospholipid distribution within the membrane is asymmetric; phosphatidylcholine and sphingomyelin are overrepresented in the outer leaflet, and phosphatidylserine (negative charge) and phosphatidylethanolamine are predominantly found on the inner leaflet; glycolipids occur only on the outer face where they contribute to the extracellular glycocalyx. Non-random partitioning of certain membrane components such as cholesterol creates membrane domains known as lipid rafts. (B) Membrane-associated proteins may traverse the membrane (singly or multiply) via α-helical hydrophobic amino acid sequences; depending on the sequence and hydrophobicity of these domain, such proteins may be enriched or excluded from lipid rafts and other membrane domain. Proteins on the cytosolic face may associate with membranes through posttranslational modifications, for example, farnesylation or addition of palmitic acid. Proteins on the extracytoplasmic face may associate with the membrane via glycosyl phosphatidyl inositol linkages. Besides protein–protein interactions within the membrane, membrane proteins can also associate with extracellular and/or intracytoplasmic proteins to generate large, relatively stable complexes (e.g., the focal adhesion complex). Transmembrane proteins can translate mechanical forces (e.g., from the cytoskeleton or ECM) as well as chemical signals across the membrane. It is worth remembering that a similar organization of lipids and associated proteins also occurs within the various organellar membranes.
segments that traverse the lipid bilayer. Integral membrane proteins typically contain positively charged amino acids in their cytoplasmic domains that anchor the proteins to the negatively charged head groups of membrane phospholipids.
Proteins may be synthesized in the cytosol and posttranslationally attached to prenyl groups (e.g., farnesyl, related to cholesterol) or fatty acids (e.g., palmitic or myristic acid) that insert into the cytosolic side of the plasma membrane.
Attachment to membranes can occur through glycosylphosphatidylinositol (GPI) anchors on the extracellular face of the membrane.
Extracellular proteins can noncovalently associate with transmembrane proteins, which serve to anchor them to the cell.