A novel RAB5 binding site in human VPS34-CII that is likely the primordial site in eukaryotic evolution

This study identifies a novel, evolutionarily primordial RAB5 binding site on the VPS15 solenoid of human VPS34 complex II, revealing that the expansion from a single ancestral binding site to two distinct sites likely enhanced membrane association and complex activity to support the development of complex eukaryotic endocytic systems.

The Gist

Imagine your cell is a bustling, high-tech city. Inside this city, there are delivery trucks (vesicles) that need to pick up packages, sort them, and deliver them to the right addresses (organelles). To keep traffic flowing, the city needs a central traffic control system.

VPS34 is that traffic control system. It's a molecular machine that puts a special "address label" (called PI3P) on the delivery trucks so they know where to go.

However, this machine doesn't run on its own. It needs a specific "key" to turn it on. In the early part of the city's history (evolution), the key was a specific type of traffic officer called RAB5.

Here is the story of what scientists discovered about how this machine works, told in simple terms:

1. The Two Versions of the Traffic Machine

The cell actually has two slightly different versions of this traffic machine:

• Version A (Complex I): Used for recycling and self-cleaning (autophagy). It listens to a different officer, RAB1.
• Version B (Complex II): Used for sorting incoming packages (endosomes). It listens to RAB5.

The scientists wanted to know exactly how Version B recognizes RAB5 and turns on.

2. The Big Surprise: Two Keys, One Lock

For a long time, scientists thought the machine had only one place where the RAB5 key could fit. They imagined it like a single keyhole on a door.

But using a super-powerful microscope (Cryo-EM), the researchers found something amazing: The machine actually has TWO keyholes.

• Keyhole #1 (The New Discovery): Located on the main body of the machine (the VPS34 part). This is the one we knew about.
• Keyhole #2 (The Ancient Secret): Located on a long, coiled arm of the machine (the VPS15 part). This was a complete surprise!

Think of it like a security guard who used to have one hand to shake hands with a VIP. Now, they have two hands. They can grab the VIP with one hand, but they also have a second hand ready to grab them again. This makes the connection much stronger and harder to break.

3. The Evolutionary Mystery: The "Primordial" Hand

The researchers dug into the history of life to see which keyhole came first.

• They looked at yeast (a simple organism), which is like the "great-grandparent" of human cells.
• They found that yeast only has the second keyhole (on the VPS15 arm). It doesn't even have the first one!
• This suggests that in the ancient past, the machine only had one hand (the VPS15 site). Over millions of years, as cells became more complex, they evolved a second hand (the VPS34 site) to hold on tighter and work faster.

It's like upgrading from a bicycle with one brake to a car with four-wheel brakes. The old brake (VPS15 site) still works and is the original design, but the new brake (VPS34 site) adds extra safety and control for the complex city of a human cell.

4. Why Having Two Hands Matters

Why does the cell need two hands to hold the key?

• Stronger Grip: Holding on with two hands makes the machine stick to the delivery trucks much better.
• Better Timing: It ensures the machine only turns on when there are lots of RAB5 officers present (a crowded traffic zone). This prevents the machine from turning on by mistake in empty areas.
• Precision: It helps position the machine perfectly on the membrane so it can do its job efficiently.

5. The "Twist" That Changes Everything

The paper also explains why Version A (the recycling machine) listens to RAB1, while Version B (the sorting machine) listens to RAB5.

• The two machines look almost identical, but they have one different part (a subunit called UVRAG in Version B and ATG14L in Version A).
• This small difference acts like a twist in the machine's shape.
• In Version B, this twist opens up the "keyhole" just enough for RAB5 to fit, but it blocks RAB1.
• In Version A, the twist is different, making a keyhole that fits RAB1 but rejects RAB5.

The Bottom Line

This paper is like finding a hidden second door on a house you thought you knew perfectly. The scientists discovered that the cell's traffic control system evolved from having one "hand" to grab its activation key, to having two hands.

This double-grip system allows human cells to manage their complex internal traffic with incredible precision, ensuring that packages are sorted and delivered exactly where they need to go. It's a beautiful example of how evolution builds on old designs (the ancient VPS15 hand) and adds new features (the VPS34 hand) to handle more complex jobs.

Technical Summary

1. Problem Statement

The Class III PI3K, VPS34, exists in two distinct mammalian complexes: Complex I (VPS34-CI) involved in autophagy and Complex II (VPS34-CII) involved in endosomal sorting. While it is established that RAB GTPases recruit and activate these complexes (RAB1A for CI, RAB5A for CII), the precise molecular mechanisms governing this specificity and the structural basis for activation were not fully resolved. Previous low-resolution cryo-electron tomography (cryo-ET) studies identified a RAB5 binding site on the VPS34 subunit of CII but failed to explain why a specific mutation (REIE>AAAA) in the VPS34 C2 domain increased RAB5 affinity in CII while abolishing it in CI. Furthermore, the existence of additional regulatory binding sites and the evolutionary origin of these interactions remained unclear.

2. Methodology

The authors employed a multi-disciplinary approach combining high-resolution structural biology, biophysics, and cell biology:

• Cryo-Electron Microscopy (Cryo-EM):
  • Determined high-resolution single-particle cryo-EM structures of human VPS34-CII bound to RAB5A-GTP.
  • Utilized specific mutants to trap different binding states:
    • REIE>AAAA: A mutant in the VPS34 C2 domain that binds RAB5 tightly.
    • REIE>ERIR: A reverse-charge mutant designed to disrupt the VPS34-RAB5 interface.
    • VPS15 Mutants: Mutations in the VPS15 solenoid domain (SHMIT>DDMIE) to disrupt the newly discovered site.
  • Performed heterogeneous refinement and 3D classification to resolve populations with 0, 1, or 2 RAB5 molecules bound.
• Biophysical Assays:
  • Confocal Microscopy Bead Assays: Measured binding affinities ($K_D$) of various VPS34-CII mutants to RAB5A.
  • GUV Kinase Assays: Assessed kinase activation on giant unilamellar vesicles containing PI3P and RAB5.
  • Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS): Validated RAB5 binding to the VPS15 solenoid in solution.
  • NanoDSF: Verified structural integrity of mutant complexes.
• Yeast Genetics and Cell Biology:
  • Generated S. cerevisiae strains with mutations in the Vps15 ortholog (SHLITY>DDLIEY).
  • Assessed endocytic sorting via Carboxypeptidase Y (CPY) processing.
  • Performed fluorescence microscopy to check colocalization of mutant Vps15 with the yeast RAB5 ortholog, Vps21.
• Computational Analysis:
  • Multiple sequence alignments across eukaryotic species to trace evolutionary conservation.
  • AlphaFold3 modeling to resolve the N-terminal zinc finger of Beclin1.

3. Key Contributions

• Discovery of a Second RAB5 Binding Site: Identification of a novel, independent RAB5 binding site on the helical solenoid domain of the VPS15 subunit (residues 570–585), distinct from the known site on the VPS34 C2 domain.
• Mechanism of Dual Binding: Demonstration that VPS34-CII can simultaneously bind two RAB5A molecules (one on VPS34, one on VPS15) and that both sites are required for full enzymatic activation.
• Structural Basis for Specificity: Elucidation of how the unique adaptor subunits (UVRAG in CII vs. ATG14L in CI) distort the tripartite binding pocket, creating a geometry that favors RAB5 over RAB1 in CII, and vice versa.
• Evolutionary Insight: Proposal that the VPS15 binding site is the "primordial" RAB5 interaction site present in early eukaryotes (like yeast), while the VPS34 site evolved later in higher organisms to enhance membrane avidity and regulation.

4. Key Results

Structural Findings

• Two Binding Sites: The high-resolution structure (3.2 Å) of the REIE>AAAA mutant revealed a second RAB5 density on the VPS15 solenoid. In wild-type (WT) VPS34-CII, 15% of particles were found to have two RAB5 molecules bound simultaneously.
• VPS15 Interaction: The VPS15 site interacts with the RAB5 hydrophobic triad (F57, W74, Y89), specifically contacting the Interswitch residue W74 and Switch 1 residue F57. This site is an $\alpha$-helix, typical of RAB effectors.
• VPS34 Interaction: The VPS34 site involves a tripartite interface (VPS15 SGD, VPS15 WD40, and VPS34 C2 helical hairpin).
• Conformational Stability: Binding of RAB5 does not induce large conformational changes in the overall VPS34-CII architecture; the complex is pre-organized for binding.

Functional Validation

• Mutant Analysis:
  • Disrupting the VPS34 site (REIE>ERIR) reduced RAB5 affinity but did not abolish activation, as the VPS15 site remained functional.
  • Disrupting the VPS15 site (SHMIT>DDMIE) reduced activation by ~2-fold.
  • Double Mutant: Simultaneous disruption of both sites completely abolished RAB5 binding and kinase activation.
• Affinity vs. Activity: While the VPS34 site has higher affinity, the VPS15 site contributes equally to activation. The authors suggest a "avidity" mechanism where binding to two sites on a membrane concentrates the complex, driving activation.

Evolutionary Context

• Yeast Conservation: The VPS34 C2 binding sequence (REIE) is not conserved in S. cerevisiae (it is reverse-charged), whereas the VPS15 binding sequence is highly conserved.
• Yeast Phenotypes: Mutating the VPS15 binding site in yeast (Vps15 SHLITY>DDLIEY) abolished binding to Vps21 (yeast RAB5), disrupted colocalization, and caused defective CPY sorting (endocytic defect).
• Conclusion: The VPS15 site is the ancestral binding site. The VPS34 site likely evolved in metazoans to provide tighter spatial/temporal control as endocytic systems became more complex.

Additional Structural Features

• Beclin1 Zinc Finger: The study resolved an N-terminal zinc finger in Beclin1 (coordinated by CXXC motifs) and the C2 domain of UVRAG, which were previously disordered in lower-resolution structures.
• Myristoylation: A density consistent with a myristoyl group on VPS15 was observed, though its functional role in activation remains debated.

5. Significance

This study fundamentally revises the understanding of VPS34 regulation:

1. Multivalency in RAB Signaling: It establishes that VPS34-CII utilizes a bivalent binding mechanism (two RAB5 molecules) to achieve high avidity and robust activation on endosomal membranes, a mechanism distinct from the monovalent binding seen in VPS34-CI.
2. Evolution of Complexity: It provides a structural and evolutionary narrative for how eukaryotic cells adapted their PI3K signaling from a single ancestral RAB-binding mechanism (on VPS15) to a dual-site system in mammals, allowing for more sophisticated regulation of endocytic trafficking.
3. Therapeutic Implications: Understanding the distinct binding interfaces of VPS34-CI and CII offers new avenues for developing specific inhibitors or modulators for diseases involving autophagy or endosomal trafficking (e.g., cancer, viral infections), as the two complexes can now be targeted based on their unique RAB-binding architectures.