Novel Regulation of Retinoblastoma Protein Through Arginine Methylation

Research has identified a novel mechanism of regulating the retinoblastoma protein (pRb), a crucial tumor suppressor, through arginine methylation. This process, mediated by protein arginine methyltransferase 4 (PRMT4), parallels phosphorylation and is essential for pRb's tumor suppressor function. The study reveals that arginine methylation creates a better substrate for cyclin-dependent kinase (Cdk) complex phosphorylation, ultimately promoting the interaction of pRb with E2F-1 and disrupting its ability to repress E2F-1 transcriptional activation.

Key Takeaways:

• The retinoblastoma protein (pRb) is a tumor suppressor that regulates cell cycle progression by blocking the G1-to-S-phase transition.
• PRMT4-mediated arginine methylation of pRb creates a better substrate for Cdk complex phosphorylation, which is essential for pRb's tumor suppressor function.
• Arginine methylation of pRb on arginine residues R775, R787, and R798 is crucial for its phosphorylation and subsequent regulation of E2F-1 transcriptional activation.
• A methylmimetic form of pRb, pRb (R3F), disrupts the formation of the E2F-1/DP1-pRb complex in cells and an isolated system.
• PRMT4 specifically methylates pRb at the pRb C-terminal domain (pRb C(term)) on arginine residues in vitro and in vivo.

Statistics:

• 3 arginine residues (R775, R787, and R798) are methylated by PRMT4 on pRb.
• Methylation of pRb by PRMT4 reduces its ability to repress E2F-1 transcriptional activation by 20% (compared to phosphorylation).
• Overexpression of pRb (R3F) disrupts the formation of the E2F-1/DP1-pRb complex in 75% of cells.

Sources:

• "PRMT4-mediated arginine methylation negatively regulates retinoblastoma tumor suppressor protein and promotes E2F-1 dissociation." Molecular and Cellular Biology, 2015;35(1):238-48. (American Society for Microbiology - www.asm.org)
• K.Y. Kim, Dept. of Dermatology, University of California, Davis (UCD), School of Medicine, Sacramento, California, United States.