Hemifusomes and Interacting Proteolipid Nanodroplets Mediate Multi-Vesicular Body Formation

Area of Science:

• Cell Biology
• Membrane Trafficking
• Organelle Biogenesis

Background:

• Classical electron microscopy struggles to visualize fragile endolysosomal intermediates.
• In situ visualization of endosomal system dynamics is limited by fixation artifacts.

Purpose of the Study:

• To investigate the in situ structure and dynamics of endolysosomal system intermediates.
• To identify novel vesicular structures and their roles in organelle biogenesis.

Main Methods:

• In situ cryo-electron tomography was employed across four mammalian cell lines.
• High-resolution imaging was used to analyze vesicular structures and their components.

Main Results:

• Discovery of a novel organelle complex termed "hemifusome," characterized by heterotypic hemifused vesicles and a proteolipid nanodroplet (PND).
• Hemifusomes constitute approximately 10% of the endolysosomal system but are not involved in transferrin-mediated endocytosis.
• Hemifusomes exhibit diverse morphologies, including compound conformations with intraluminal vesicles, suggesting a role in vesicular biogenesis.

Conclusions:

• Hemifusomes represent a previously undescribed organelle complex within the endolysosomal system.
• The proteolipid nanodroplet (PND) is proposed to mediate vesicular biogenesis on hemifusome platforms.
• A new, ESCRT-independent model for the formation of late endosomes and multivesicular bodies (MVBs) is presented, involving long-lived hemifusion diaphragms.