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Phosphoinositides are a group of phospholipids containing a glycerol backbone with two fatty acid chains and a phosphate attached to a myoinositol sugar ring. The inositol head group extends into the cytoplasm, where it is modified by adding phosphate groups to form phosphatidylinositol phosphates or PIPs.
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Rab proteins constitute the largest family of monomeric GTPases, of which 70 members are present in humans. Rab proteins and their effectors regulate consecutive stages of vesicle transport such as vesicle transport, docking, and fusion to the correct recipient membrane.
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Rab GTPases act in a regulated cascade during membrane fusion, helping the lipid bilayers mix. The Rab family of proteins are active when bound to GTP, and inactive when bound to GDP. Hence, they act as guanine nucleotide-dependent molecular switches. Rab-GTP recognizes and binds to long or short-range tethering proteins to capture the target vesicle. These tethers coordinate with SNAREs on the vesicle and the target membrane to assemble the trans SNARE complex that locks the mixing bilayers.
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Essential proteins such as insulin or low-density lipoprotein (LDL) and micronutrients such as iron enter a eukaryotic cell through receptor-mediated endocytosis. Subsequently, the early endosomes fuse with the vesicles containing such receptor-ligand complexes and play a vital role in sorting the incoming ligands and receptors. While the ligands are either degraded inside the vesicle or released into the cytosol, their receptors are returned to the plasma membrane for further rounds of...
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The recycling endosome, also known as the endosomal recycling compartment (ERC), is a part of the slow-recycling process of the endocytic pathway. Molecules internalized through receptor-mediated endocytosis are either degraded in the lysosomes or are recycled to the plasma membrane through the fast- or slow-recycling route.
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Updated: Feb 22, 2026

Fluorescence-Based Measurements of Phosphatidylserine/Phosphatidylinositol 4-Phosphate Exchange Between Membranes
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A new Rab7 effector controls phosphoinositide conversion in endosome maturation.

James E Casanova1, Bettina Winckler2

  • 1Department of Cell Biology, University of Virginia, Charlottesville, VA.

The Journal of Cell Biology
|September 21, 2017
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WDR91 is a newly identified protein that acts as a Rab7 effector. It inhibits phosphatidylinositol 3-kinase, which is crucial for endosome maturation, cell survival, and neuron development.

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Area of Science:

  • Cell Biology
  • Molecular Biology
  • Neuroscience

Background:

  • Endosome maturation involves coordinated changes in Rab GTPases and phosphoinositides.
  • Rab GTPases regulate various membrane trafficking events, including endosomal transport.

Purpose of the Study:

  • To identify novel regulators of endosome maturation.
  • To investigate the role of WDR91 in endosomal trafficking and neuronal function.

Main Methods:

  • Yeast two-hybrid screening to identify Rab7 interacting proteins.
  • Biochemical assays to measure phosphatidylinositol 3-kinase activity.
  • In vivo studies in neurons to assess dendrite growth and viability.

Main Results:

  • WDR91 was identified as a ubiquitous Rab7 effector.
  • WDR91 inhibits phosphatidylinositol 3-kinase (PI3K) activity on endosomes.
  • WDR91 is essential for endosome maturation, neuronal viability, and dendrite growth in vivo.

Conclusions:

  • WDR91 is a critical regulator of endosome maturation by controlling PI3K activity.
  • WDR91 plays a vital role in neuronal development and survival.