12.9
Lipidi come ancore
Nella membrana plasmatica, i lipidi che formano il doppio strato possono anche fungere da ancoraggio per legare le proteine alla membrana. I tre princ…
I tre tipi più comuni di ancore lipidiche sono i gruppi prenilici, i gruppi acilici grassi e le ancore glicosilfosfatidilinositolo o GPI.
I gruppi prenilici sono i gruppi farnesile a 15 atomi di carbonio e i gruppi geranilgeranilici a 20 atomi di carbonio. Si legano ai residui di cisteina in corrispondenza o in prossimità dell'estremità carbossilica delle proteine.
L'acido miristico saturo a 14 atomi di carbonio e l'acido palmitico a 16 atomi di carbonio sono le ancore aciliche grasse più comuni legate alle proteine.
L'aggiunta di acido miristico, o miristoilazione, avviene al residuo di glicina N-terminale di una proteina. La palmitoilazione delle proteine può avvenire a livello del residuo di cisteina N- o C- terminale.
Leproteine prenilate, miristoilate e palmitoilate svolgono un ruolo essenziale nelle vie di segnalazione intracellulare e nelle interazioni proteina-proteina.
Le ancore GPI si legano alle proteine extracellulari e contengono una struttura centrale di fosfatidilinositolo, glucosamina, tre mannosio e fosfoetanolammina.
La fosfoetanolammina si lega all'aminoacido C-terminale della proteina bersaglio e il fosfolipide si inserisce nello strato esterno della membrana.
Ciò consente alle proteine ancorate a GPI di partecipare a funzioni extracellulari come la comunicazione cellula-cellula e l'adesione cellulare.
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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.