Phosphatidylinositol diphosphate binding by ESCRT-III filaments

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Inositol phospholipids (PIPs) act as organelle-specific signals, guiding PIP-binding proteins like the Endosomal Sorting Complexes Required for Transport (ESCRT) machinery.
• The ESCRT pathway, involving ESCRT-I, ESCRT-II, and ESCRT-III complexes, is crucial for membrane remodeling, constriction, and fission events.
• ESCRT-III filaments, along with VPS4 ATPases, mediate the final stages of membrane remodeling.

Purpose of the Study:

• To elucidate the structural mechanisms by which ESCRT-III filaments interact with phosphoinositide-2 (PIP2) lipids.
• To investigate how different PIP2 isomers influence the assembly and function of ESCRT-III polymers.
• To understand the role of PIP2 in promoting ESCRT-III filament formation and membrane tubulation.

Main Methods:

• Cryo-electron microscopy (Cryo-EM) reconstructions of ESCRT-III filaments.
• Structural analysis of IST1 and CHMP1A protofilaments in complex with PIP2-containing membranes.
• Biochemical characterization of ESCRT-III/PIP2 interactions.

Main Results:

• Two distinct classes of helical ESCRT-III filaments were identified: IST1 forming 8-stranded nanotubes and CHMP1A forming one-start helices.
• IST1 nanotubes encase membrane monolayers, coordinating PIP2 headgroups within a specific basic pocket.
• CHMP1A helices encase membrane bilayers, binding PIP2 headgroups across a basic surface spanning adjacent subunits.

Conclusions:

• The findings reveal the structural plasticity of ESCRT-III polymers in interacting with membranes.
• PIP2 lipids are shown to promote ESCRT-III filament assembly and membrane remodeling.
• Specific molecular contacts between ESCRT-III subunits and PIP2 lipids are defined, explaining interaction specificity.