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Redefining PH Domain Function: An Active Allosteric Mechanism in ASAP1-Mediated Arf1 GTP Hydrolysis.

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    Area of Science:

    • Molecular Biology
    • Biochemistry
    • Structural Biology

    Background:

    • GTPase-activating proteins (GAPs) are crucial regulators of small GTPases, influencing diverse cellular processes.
    • ASAP1, a GAP, activates GTP hydrolysis on Arf1 and is linked to cancer progression.
    • ASAP1's Pleckstrin Homology (PH) domain is essential for its Arf1 GTPase-activating protein (GAP) activity.

    Purpose of the Study:

    • To investigate the regulatory mechanism of the ASAP1 PH domain on Arf1 GTP hydrolysis.
    • To challenge the prevailing view of PH domains acting solely through passive membrane recruitment.
    • To elucidate the active role of the ASAP1 PH domain in modulating Arf1 activity.

    Main Methods:

    • Nuclear Magnetic Resonance (NMR) spectroscopy
    • Molecular dynamics (MD) simulations
    • Kinetic assays
    • Mutational analysis
    • Mathematical modeling

    Main Results:

    • The ASAP1 PH domain actively interacts with Arf·GTP at the membrane, inducing conformational changes in Arf1's GTP binding site.
    • These structural rearrangements stabilize the transition state for GTP hydrolysis, significantly increasing the catalytic rate.
    • Key residues within the PH domain and Arf1 were identified as critical for this allosteric regulation.
    • The allosteric mechanism contributes equally to ASAP1's GAP activity as membrane recruitment.

    Conclusions:

    • PH domains can actively regulate small GTPase activity beyond passive membrane targeting.
    • This discovery reveals a novel allosteric mechanism for GTPase regulation by ASAP1.
    • The findings have broad implications for understanding the regulation of other small GTPases, including Ras and Rho proteins, by PH domain-containing proteins.