GTP hydrolysis is not important for Ypt1 GTPase function in vesicular transport

Area of Science:

• Cell Biology
• Molecular Biology
• Protein Function

Background:

• Ypt/Rab GTPases regulate vesicular transport by cycling between GTP- and GDP-bound states.
• GTP hydrolysis, often stimulated by GTPase-activating protein (GAP), is thought to be crucial for vesicle fusion and timing.
• Current models posit GTP hydrolysis is essential for terminating Ypt1p function in membrane fusion.

Purpose of the Study:

• To investigate the necessity of GTP hydrolysis for Ypt1p function in vesicular transport.
• To challenge existing models regarding the role of GTP hydrolysis in regulating Ypt1p-mediated membrane fusion.
• To explore the function of Ypt1p in relation to its nucleotide-binding state and interactions.

Main Methods:

• Utilized a Q67L mutation in Ypt1p to inactivate its GTPase activity.
• Assessed cellular growth phenotypes, secretion rates, and membrane morphology in yeast strains expressing the mutant Ypt1p.
• Examined the effect of dominant nucleotide-free YPT1 mutations on Ypt1p function.

Main Results:

• The Ypt1-Q67L mutant protein was severely impaired in GTP hydrolysis, remaining predominantly GTP-bound.
• Yeast cells expressing only the Ypt1-Q67L mutant exhibited no growth defects or altered secretion/membrane morphology across a range of temperatures.
• Overexpression of Ypt1-Q67L did not cause dominant negative phenotypes.
• The Ypt1-Q67L allele, similar to wild-type, was inhibited by dominant nucleotide-free YPT1 mutations, suggesting GNEF interaction is vital.

Conclusions:

• GTP hydrolysis is not essential for Ypt1p-mediated vesicular transport or as a timer for membrane fusion.
• GTP hydrolysis is primarily required for the recycling of Ypt1p between cellular compartments.
• Ypt1p function in vesicular transport depends on both its GTP-bound state and its interaction with guanine nucleotide exchange factor (GNEF).