A structure-based mechanism for initiation of AP-3 coated vesicle formation

Area of Science:

• Cell biology
• Structural biology
• Molecular trafficking

Background:

• Adaptor protein complex-3 (AP-3) sorts cargo to lysosomes and related organelles.
• Unlike AP-1 and AP-2, AP-3 exists in an open conformation, raising questions about its regulation.
• Understanding AP-3 activation is crucial for elucidating vesicular transport mechanisms.

Purpose of the Study:

• To determine the mechanism of AP-3 membrane recruitment and activation.
• To elucidate the structural changes involved in AP-3 coated vesicle formation.
• To link cargo sorting with the assembly of AP-3 coat structures.

Main Methods:

• Reconstitution of human AP-3 and Arf1 complexes.
• Electron cryo-microscopy (cryo-EM) to determine structures in soluble and membrane-bound states.
• Utilized lipid nanodiscs for membrane-bound complex formation.

Main Results:

• Human AP-3 maintains a constitutively open conformation.
• AP-3 membrane recruitment is mediated by Arf1 binding.
• Cargo binding stabilizes AP-3, facilitating a second Arf1 interaction and initiating dimerization/polymerization.
• Identified amphipathic helices in AP-3, suggesting a role in membrane deformation.

Conclusions:

• AP-3 activation is a stepwise process involving Arf1 and cargo binding.
• Coat polymerization is triggered by cargo engagement, connecting sorting with coat assembly.
• AP-3 directly contributes to membrane deformation during vesicle formation.
• These findings reveal the initial stages of AP-3-mediated vesicle coat assembly.