SRP RNA provides the physiologically essential GTPase activation function in cotranslational protein targeting

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Signal recognition particle (SRP) mediates cotranslational protein targeting to cell membranes.
• The SRP targeting cycle is regulated by GTP binding and hydrolysis involving the SRP and its receptor.
• The mechanism stimulating GTP hydrolysis in vivo remained unclear due to the absence of known GTPase-activating proteins (GAPs).

Purpose of the Study:

• To investigate the role of SRP RNA in stimulating GTPase activity of the SRP-receptor complex.
• To identify the specific RNA element responsible for GTPase stimulation.
• To elucidate the mechanism by which SRP RNA facilitates GTP hydrolysis during protein trafficking.

Main Methods:

• Biochemical assays to measure GTPase activity.
• Genetic experiments using SRP RNA mutants.
• Tethered hydroxyl radical probing to determine RNA positioning.

Main Results:

• SRP RNA significantly enhances the GTPase activity of the SRP-receptor complex.
• The SRP RNA tetraloop is the key element responsible for this GTPase stimulation.
• Mutations in the SRP RNA tetraloop impair GTP hydrolysis and cell growth.
• Specific positioning of the tetraloop within the complex is critical for efficient GTPase activity.

Conclusions:

• The SRP RNA tetraloop acts as an intrinsic GTPase-activating element for the SRP-receptor complex.
• This RNA-mediated stimulation explains why external GAPs are not required for SRP function.
• The conserved SRP RNA tetraloop is essential for efficient protein targeting and cell viability.