12.9
Lipide als Anker
In der Plasmamembran können die Lipide, die die Doppelschicht bilden, auch als Anker fungieren, um Proteine an die Membran zu binden. Die drei Hauptty…
Die drei gebräuchlichsten Arten von Lipidankern sind Prenylgruppen, Fettacylgruppen und Glykosylphosphatidylinositol- oder GPI-Anker.
Die Prenylgruppen sind die 15 Kohlenstoff-Farnesyl- und die 20 Kohlenstoff-Geranylgeranyl-Gruppen. Sie binden an Cysteinreste am oder in der Nähe des Carboxy-Endes von Proteinen.
Gesättigte Myristinsäure mit 14 Kohlenstoffatomen und Palmitinsäure mit 16 Kohlenstoffatomen sind die häufigsten Fettacylanker, die mit Proteinen verbunden sind.
Die Zugabe von Myristinsäure oder Myristoylierung erfolgt am N-terminalen Glycinrest eines Proteins. Die Palmitoylierung der Proteine kann am N- oder C-terminalen Cysteinrest erfolgen.
Prenylierte, myristoylierte und palmitoylierte Proteine spielen eine wesentliche Rolle bei intrazellulären Signalwegen und Protein-Protein-Interaktionen.
GPI-Anker binden an extrazelluläre Proteine und enthalten eine Kernstruktur aus Phosphatidylinositol, Glucosamin, drei Mannosen und Phosphoethanolamin.
Das Phosphoethanolamin bindet sich an die C-terminale Aminosäure des Zielproteins, und das Phospholipid fügt sich in die äußere Schicht der Membran ein.
Dies ermöglicht es GPI-verankerten Proteinen, an extrazellulären Funktionen wie Zell-Zell-Kommunikation und Zelladhäsion teilzunehmen.
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Q1: What are the three main types of lipid anchors found in eukaryotic membranes?
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.
Q2: How do prenyl groups attach to proteins and what determines the type of prenylation?
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.
Q3: What is the difference between myristoylation and palmitoylation?
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.
Q4: How do GPI anchors differ from prenyl and fatty acyl anchors in their membrane orientation?
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.
Q5: What cellular functions do lipid-anchored proteins perform?
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.
Q6: Why do some proteins require multiple lipid anchors?
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.
Q7: What is the CaaX motif and how does it determine prenylation specificity?
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.