Plexin-Bs enhance their GAP activity with a novel activation switch loop generating a cooperative enzyme

Area of Science:

• Cell biology
• Molecular signaling
• Biochemistry

Background:

• Plexins are single-pass transmembrane receptors mediating cell guidance via semaphorin ligands.
• Their intracellular regions interact with small GTPases, regulating cytoskeleton and cell adhesion.
• The GTPase Activating Protein (GAP) function of plexins is crucial for signal transduction.

Purpose of the Study:

• To characterize the GTPase Activating Protein (GAP) function of Plexin-B1.
• To identify the molecular mechanism underlying Plexin-B1 activation.
• To explore the biological implications of Plexin-B1's cooperative activity in cell migration.

Main Methods:

• Biochemical assays to characterize GTPase Activating Protein (GAP) activity.
• Mutagenesis studies to identify functional regions and residues.
• Computational simulations to model protein-ligand interactions.
• Analysis of cell migration behavior.

Main Results:

• Plexin-B1 displays cooperative GAP activity towards the small GTPase Rap1b.
• This cooperative activity is localized to the N-terminal Juxtamembrane (JM) region.
• A novel functional loop was identified that regulates Rap1b binding to the plexin GAP domain.
• Substrate-mediated cooperativity enhances enzymatic turnover and contributes to threshold behavior in cell migration.

Conclusions:

• Plexin-B1's GAP activity is regulated by a novel activation mechanism involving a functional loop and JM region.
• This mechanism leads to cooperative substrate binding and enhanced enzymatic turnover.
• The findings provide insights into the regulation of cell migration through threshold responses mediated by Plexin-B1.