AP-3 shows off its flexibility for the cryo-EM camera

Area of Science:

• Cellular Biology
• Protein Structure and Dynamics
• Membrane Trafficking

Background:

• The tetrameric adaptor protein AP-3 is essential for protein sorting to lysosomes and related organelles.
• Homologous adaptor proteins AP-1 and AP-2 undergo conformational changes upon membrane binding.
• Previous studies on AP-1 and AP-2 highlight a closed-to-open transition regulated by membrane association and phosphoinositides.

Purpose of the Study:

• To investigate the structural conformation of the tetrameric adaptor protein AP-3.
• To compare the conformational flexibility of AP-3 with that of AP-1 and AP-2.
• To understand the implications of AP-3's structure on its membrane recruitment mechanism.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine the structures of AP-3 from budding yeast.
• Solution structures were analyzed to assess conformational flexibility.
• Comparative analysis with existing structural data of AP-1 and AP-2 was performed.

Main Results:

• The first cryo-EM structures of AP-3 from budding yeast were reported.
• AP-3 exhibited remarkably flexible solution structures, consistently adopting an open conformation.
• This contrasts with the known closed-to-open transition of AP-1 and AP-2, suggesting AP-3 lacks a stable closed state.

Conclusions:

• AP-3 displays significant conformational flexibility, predominantly existing in an open state.
• The absence of a readily available closed state may make AP-3 more dependent on cargo interactions for initial membrane recruitment compared to AP-1.
• These findings provide new insights into the distinct mechanisms of protein transport mediated by different adaptor protein complexes.