PI4P and BLOC-1 remodel endosomal membranes into tubules

Abstract

Intracellular trafficking is mediated by transport carriers that originate by membrane remodeling from donor organelles. Tubular carriers contribute to the flux of membrane lipids and proteins to acceptor organelles, but how lipids and proteins impose a tubular geometry on the carriers is incompletely understood. Using imaging approaches on cells and in vitro membrane systems, we show that phosphatidylinositol-4-phosphate (PI4P) and biogenesis of lysosome-related organelles complex 1 (BLOC-1) govern the formation, stability, and functions of recycling endosomal tubules. In vitro, BLOC-1 binds and tubulates negatively charged membranes, including those containing PI4P. In cells, endosomal PI4P production by type II PI4-kinases is needed to form and stabilize BLOC-1-dependent recycling endosomal tubules. Decreased PI4KIIs expression impairs the recycling of endosomal cargoes and the life cycles of intracellular pathogens such as Chlamydia bacteria and influenza virus that exploit the membrane dynamics of recycling endosomes. This study demonstrates how a phospholipid and a protein complex coordinate the remodeling of cellular membranes into functional tubules.

Type II phosphatidylinositol 4-kinases function sequentially in cargo delivery from early endosomes to melanosomes

“…2 B ). These results suggest that while depletion of either PI4KIIα or PI4KIIβ partially traps TYRP1 in early endosomes, consistent with a redundant requirement for either kinase to initiate BLOC-1-dependent cargo export from endosomes ( Jani et al, 2022 ), neither alone is absolutely required for BLOC-1-dependent cargo exit from early sorting endosomes.…”

“…Here, we used gene silencing together with imaging and biochemical approaches to test whether PI4KIIα and/or PI4KIIβ function in BLOC-1-dependent cargo transport to melanosomes by generating PtdIns4P on endosomal tubules. While PI4KIIα and PI4KIIβ likely function redundantly in BLOC-1-dependent tubule initiation ( Jani et al, 2022 ), we show that they play additional non-redundant roles in generating PtdIns4P on the tubular carriers in melanocytes. This is necessary for tubule stability and for cargo delivery to melanosomes.…”

“…Because TYRP1 was trapped less in early endosomes upon PI4KIIα or PI4KIIβ depletion than upon BLOC-1 depletion/knockout, we hypothesized that while either PI4KIIα or PI4KIIβ can collaborate with BLOC-1 to facilitate cargo exit from early sorting endosomes ( Jani et al, 2022 ), each additionally promotes an ensuing step in tubule stabilization and/or targeting toward melanosomes. If so, then TYRP1 should remain trapped in early endosomes and not progress to late endosomes/lysosomes upon PI4KIIα or PI4KIIβ depletion in BLOC-1-deficient cells.…”

“…One of these pathways uniquely requires BLOC-1 to generate recycling-endosome-like tubular transport intermediates from early sorting endosomes that ultimately fuse with melanosomes, delivering cargoes such as tyrosinase-related protein-1 (TYRP1) and the melanosomal chloride channel ( Bellono and Oancea, 2014 ; Sitaram et al, 2009 ) oculocutaneous albinism type 2 (OCA2; Delevoye et al, 2009 ; Setty et al, 2008 ; Setty et al, 2007 ; Sitaram et al, 2012 ). BLOC-1 coordinates with the microtubule motor KIF13A and actin polymerization to generate the tubules ( Delevoye et al, 2016 ; Delevoye et al, 2009 ) and may directly stabilize them ( Jani et al, 2022 ; Lee et al, 2012 ). Adaptor protein-1 (AP-1) and a BLOC-1-associated cohort of AP-3 sort cargoes and SNARE proteins (including the v-SNARE, VAMP7) into the melanosome-bound tubules ( Bowman et al, 2021 ; Delevoye et al, 2009 ; Dennis et al, 2015 ; Theos et al, 2005 ).…”

“…Similarly, PI4KIIβ binds directly to AP-1 ( Wieffer et al, 2013 ). Moreover, BLOC-1 binds to PtdIns4P and other phosphoinositides on tubular membranes in vitro, and requires type II PtdIns-4-kinases to initiate recycling endosome formation in HeLa cells ( Jani et al, 2022 ). Here, we used gene silencing together with imaging and biochemical approaches to test whether PI4KIIα and/or PI4KIIβ function in BLOC-1-dependent cargo transport to melanosomes by generating PtdIns4P on endosomal tubules.…”