Mechanism and cellular function of direct membrane binding by the ESCRT and ERES-associated Ca²⁺-sensor ALG-2

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Apoptosis Linked Gene-2 (ALG-2) is a Ca²⁺-sensitive protein involved in membrane repair and protein transport.
• ALG-2 interacts with proteins like ESCRT-I, ALIX, and SEC31A, and binds to acidic membranes in a Ca²⁺-dependent manner.
• Understanding the direct membrane interactions of ALG-2 is key to elucidating its diverse cellular roles.

Approach:

• Utilized giant unilamellar vesicles (GUVs) and molecular dynamics (MD) simulations to investigate ALG-2's membrane binding.
• Created charge-reversed mutants of ALG-2 to probe the role of specific charged residues in membrane recruitment.
• Employed in vitro reconstitution assays to assess the impact of protein interactions on ALG-2 membrane binding.

Key Points:

• Direct Ca²⁺-dependent membrane binding by ALG-2 involves both electrostatic and hydrophobic interactions.
• Mutations disrupting ALG-2's direct membrane binding impair its localization to endoplasmic reticulum exit sites (ERES) but not lysosomes.
• ALG-2's membrane-binding defect can be rescued by its interaction with ESCRT-I, highlighting the interplay between direct membrane and protein binding.

Conclusions:

• The study reveals the molecular basis of direct Ca²⁺-dependent membrane binding by ALG-2.
• Demonstrates the differential requirement of direct membrane binding for ALG-2's localization to ERES versus lysosomes.
• Establishes that ALG-2's cellular functions are regulated by a combination of direct membrane interactions and protein-binding partners.