Homotypic SCOTIN assemblies form ER-endosome membrane contacts and regulate endosome dynamics

Area of Science:

• Cell Biology
• Membrane Trafficking
• Organelle Dynamics

Background:

• The endoplasmic reticulum (ER) organizes endolysosomal systems via membrane contact.
• Existing ER-endosome tethering relies on heterotypic interactions.
• A novel mechanism for ER-endosome tethering is investigated.

Purpose of the Study:

• To identify and characterize a novel ER-endosome tethering mechanism.
• To elucidate the role of SCOTIN protein in ER-endosome membrane contact.
• To define the functional domains of SCOTIN involved in tethering.

Main Methods:

• SCOTIN protein localization studies in ER and endosomes.
• Analysis of SCOTIN-knockout (KO) cells for ER-late endosome contacts and endosome positioning.
• In vitro studies of SCOTIN proline-rich domain (PRD) assembly and liposome tethering.
• Functional assays using reconstituted SCOTIN in KO cells and chimeric PRD domains.

Main Results:

• SCOTIN is a single-pass transmembrane protein found on ER and endosome membranes.
• SCOTIN deficiency (KO) reduces ER-late endosome contacts and disrupts endosome positioning.
• The SCOTIN PRD self-assembles and is essential for ER-endosome tethering.
• A specific 28-amino acid region (150-177) within the PRD is critical for tethering and endosomal dynamics.
• Purified SCOTIN PRD can mediate liposome tethering in vitro.
• SCOTIN assembly on both ER and endosome membranes is required for organelle tethering.

Conclusions:

• SCOTIN mediates ER-endosome tethering through homotypic interactions of its PRD.
• The SCOTIN PRD's ability to assemble is sufficient for membrane tethering.
• SCOTIN plays a key role in maintaining ER-endosome spatial organization and endosomal trafficking.