Farnesylation is a fundamental biological process involving the modification of proteins within cells. This post-translational modification attaches a farnesyl group to specific proteins after they are synthesized. These modifications are widespread and necessary for many proteins to function correctly and participate in cellular signaling. The farnesyl group helps position proteins accurately to carry out their roles.
The Molecular Machinery of Farnesylation
Farnesylation involves the covalent attachment of a farnesyl group, a hydrophobic 15-carbon lipid molecule, to a protein. This modification occurs at a cysteine residue, an amino acid containing sulfur, near the end of the protein chain, forming a stable thioether linkage. Farnesyltransferase (FTase) is the enzyme responsible for catalyzing this reaction. FTase transfers the farnesyl group from farnesyl diphosphate (FPP) to the target protein.
FTase recognizes target proteins based on specific amino acid sequences, primarily the C-terminal CaaX motif. This four-amino acid sequence consists of a cysteine (C), two aliphatic amino acids (aa), and a variable terminal amino acid (X). The identity of the ‘X’ amino acid determines whether the protein is farnesylated or undergoes geranylgeranylation. If the X position is methionine (M), serine (S), glutamine (Q), alanine (A), or cysteine (C), FTase will farnesylate the protein.
Recent discoveries show FTase can also farnesylate proteins with longer C(x)3X motifs, indicating a more flexible active site. This potentially increases the number of proteins that can undergo farnesylation within human cells. After farnesylation, additional processing steps occur, including the proteolytic removal of ‘aaX’ residues by an enzyme called RAS-converting CAAX endopeptidase 1 (RCE1), followed by carboxylmethylation of the farnesylated cysteine.
Cellular Roles of Farnesylation
Farnesylation plays an important role in ensuring proteins are correctly positioned and function within cells. The farnesyl group’s hydrophobic nature allows it to act as a lipid anchor, facilitating the attachment of modified proteins to cellular membranes, such as the plasma membrane. This membrane association is often a prerequisite for a protein’s biological activity, as many signaling pathways occur at these membranes.
One of the most well-known examples of farnesylated proteins are members of the Ras superfamily of small GTP-binding proteins. These proteins are involved in many cellular processes, including regulating cell growth, differentiation, and survival. Farnesylation of Ras proteins is necessary for their activation and localization to the cell membrane. Once anchored, Ras proteins interact with other signaling molecules, transmitting signals that control various cellular functions. Without farnesylation, Ras proteins cannot properly associate with membranes and are unable to relay signals effectively.
Farnesylation’s Role in Health and Disease
Dysregulation of farnesylation can contribute to the development and progression of various diseases. Its most studied connection is its involvement in cancer. Oncogenic forms of Ras proteins, often resulting from mutations, are frequently farnesylated. This farnesylation promotes their association with cell membranes, enabling them to continuously activate signaling pathways that drive uncontrolled cell proliferation and survival, hallmarks of cancer.
Beyond cancer, farnesylation pathways are recognized for their implications in neurodegenerative conditions, particularly Alzheimer’s disease (AD). Studies indicate that protein farnesylation is elevated in the brains of individuals with Alzheimer’s. Small GTPases, which are often farnesylated, regulate diverse cellular processes like cell cycle entry and cell adhesion. Dysregulation of these proteins has been linked to various disorders, including progeria and neurodegenerative diseases. Upregulation of farnesyltransferase and subsequent protein farnesylation is an early factor in Alzheimer’s disease, with overactivation of downstream signaling pathways contributing to cognitive impairment and neuropathology.
Targeting Farnesylation for Therapy
Understanding farnesylation’s role in disease has spurred the development of therapeutic strategies, particularly in oncology. Farnesyltransferase Inhibitors (FTIs) are drugs designed to block the farnesylation process. The main goal of FTIs was to inhibit the farnesylation of oncogenic Ras proteins, preventing their membrane localization and activation. By interfering with this lipid modification, FTIs aim to disrupt signaling pathways that drive uncontrolled cell growth in cancer.
Preclinical studies showed FTIs could inhibit tumor growth with limited effects on normal cell physiology. However, early clinical trials yielded disappointing results, revealing a gap in understanding FTI activity. This was partly due to the discovery that some Ras isoforms, like KRAS and NRAS, could undergo alternative lipid modifications in the presence of FTIs, still allowing them to associate with membranes and function. Despite these challenges, FTIs have shown promise in treating certain non-neoplastic diseases, such as Hutchinson-Gilford progeria syndrome, and have been investigated for other conditions like diabetic retinopathy and macular degeneration.