Lipids as Anchors

The three main types are prenyl groups, fatty acyl groups, and glycosylphosphatidylinositol (GPI) anchors. Prenyl and fatty acyl groups anchor proteins on the cytosolic surface of the membrane, while GPI anchors attach proteins to the extracellular side. Each type uses different chemical linkages and serves distinct cellular functions.

Prenyl groups bond to cysteine residues at or near the protein's carboxy terminus through a CaaX motif. The amino acid at the X position determines whether farnesyl transferase attaches a 15-carbon farnesyl group or geranylgeranyl transferase attaches a 20-carbon geranylgeranyl group. After prenylation, proteases remove the tripeptide, and methylation makes the cysteine hydrophobic.

Myristoylation adds a 14-carbon myristic acid to a protein's N-terminal glycine residue, while palmitoylation adds a 16-carbon palmitic acid to N- or C-terminal cysteine residues. Both are saturated fatty acyl anchors that tether proteins to the membrane. Proteins can use both modifications simultaneously for stronger membrane attachment.

GPI anchors attach to extracellular proteins on the outer membrane layer, whereas prenyl and fatty acyl anchors position proteins on the cytosolic surface. GPI anchors contain a core structure of phosphatidylinositol, glucosamine, three mannoses, and phosphoethanolamine, with the phospholipid inserting into the bilayer and the phosphoethanolamine bonding to the protein's C-terminal amino acid.

Prenylated, myristoylated, and palmitoylated proteins play essential roles in intracellular signaling pathways and protein-protein interactions. GPI-anchored proteins participate in extracellular functions such as cell-cell communication and cell adhesion. These lipid anchors enable proteins to interact with membrane lipids and other membrane components effectively.

Some proteins use more than one lipid anchor to achieve stronger and more stable membrane attachment. For example, cytoplasmic tyrosine kinase is anchored using both myristic and palmitic acid simultaneously. However, GPI-anchored proteins typically do not require additional anchors because their two fatty acyl groups and phosphoethanolamine linkage provide sufficient stabilization.

The CaaX motif is a recognition sequence at the carboxy-terminal of prenylated proteins where C is cysteine, a represents any aliphatic amino acid, and X is any other amino acid. If X is alanine, serine, methionine, cysteine, or glutamine, farnesyl transferase recognizes it and attaches farnesyl. If X is glutamic acid or leucine, geranyl transferase type I attaches geranylgeranyl instead.