Septin-mediated coupling of protein import and division during chloroplast evolution

This study reveals that the algal septin SEP1 physically couples chloroplast protein import and organelle division by linking TOC GTPases to the division machinery, suggesting this coordination mechanism emerged early in chloroplast evolution.

The Gist

The Big Picture: The Chloroplast's "Construction Site"

Imagine a plant cell as a busy city. Inside this city are chloroplasts, which are like solar power plants that generate energy for the city. For the city to grow, these power plants need to do two things simultaneously:

1. Import Supplies: They need to bring in new parts (proteins) built in the city center (the nucleus) to keep running.
2. Divide: When the city gets too big, the power plants need to split in half so every new neighborhood gets one.

For a long time, scientists thought these two jobs were handled by separate crews working independently. This new paper discovers that they are actually managed by a single, ancient "foreman" named SEP1.

Meet the Foreman: SEP1

In most animals and fungi (like yeast), the crew that manages cell division is made of many different workers called septins. They form a ring around the neck of a dividing cell to squeeze it in two, like a rubber band tightening around a balloon.

However, the green alga Chlamydomonas (a single-celled plant) is different. It only has one septin gene, named SEP1. The researchers found that this single foreman does a double-duty job that no one expected:

1. The Import Inspector: During the day (interphase), SEP1 forms a net-like fence on the outside of the chloroplast. It acts like a bouncer at a club, helping specific "division proteins" get inside the chloroplast.
2. The Squeeze Ring: When it's time to divide (during cell division), this net reorganizes into a tight ring right in the middle of the chloroplast, helping to pinch it in two.

The Discovery: A Broken Chain of Command

To figure out what SEP1 actually does, the scientists built a version of the alga where they removed the SEP1 gene (a "mutant").

• The Result: The mutant algae looked mostly normal at first. They could grow and photosynthesize.
• The Stress Test: When the scientists stressed the cells (by messing with their internal skeleton), the mutants fell apart. Their chloroplasts became misshapen, lumpy, and sometimes failed to divide.
• The Missing Link: The researchers realized that without SEP1, the chloroplast couldn't get the specific proteins it needed to divide. It was like a construction site where the general supplies arrived, but the specialized tools for building the new half of the building were stuck at the gate.

The Secret Connection: The "G-Interface"

How does SEP1 know which proteins to let in? It turns out SEP1 has a special handshake with the chloroplast's main gate, called the TOC complex.

• The Analogy: Imagine the TOC complex is the main door to the chloroplast. SEP1 is the security guard standing right next to it.
• The Twist: The researchers found that the "handshake" (a specific molecular shape called the G-domain) that SEP1 uses to talk to the door is almost identical to the handshake the door uses to talk to itself.
• The Evolutionary Shock: By looking at the family tree of these proteins, the scientists realized something amazing: The chloroplast's door (TOC) actually evolved from an ancient version of the septin foreman (SEP1).

It's as if the security guard (SEP1) and the door (TOC) are long-lost cousins. They look different now, but they share the same DNA blueprint. This suggests that billions of years ago, when bacteria first became chloroplasts, the host cell used its own "septin" proteins to help manage the new intruder, and eventually, those proteins evolved into the very doors that let proteins in.

The Time Travel Experiment

To prove this ancient connection was real, the scientists played a game of "evolutionary time travel."

1. The Test: They took the SEP1 protein from the green alga (which has it) and forced it to work inside land plants like moss, Arabidopsis, and tobacco (which lost their septins millions of years ago).
2. The Result: Even though land plants don't have septins naturally, the algal SEP1 protein jumped right in, found the chloroplasts, and started forming rings and filaments just like it did in the alga.
3. The Meaning: This proves that the "instructions" for how to manage the chloroplast are so deeply embedded in our evolutionary history that a protein from an ancient alga can still talk to the chloroplasts of a modern flower.

The Takeaway

This paper tells a story of ancient teamwork.

• Before: We thought protein import and cell division were separate processes.
• Now: We know they are coordinated by a single protein (SEP1) that acts as a bridge.
• The Evolution: The machinery that lets proteins into the chloroplast (the TOC complex) actually evolved from the same family of proteins that helps squeeze cells in half (septins).

In short, the "bouncer" at the chloroplast door and the "squeezing ring" that divides the chloroplast are part of the same ancient family, working together to ensure that the plant's solar power plants are built correctly and split evenly. It's a perfect example of how evolution repurposes old tools to build new, complex systems.

Technical Summary

1. Problem Statement

Chloroplast biogenesis relies on two critical, coordinated processes: the import of nuclear-encoded proteins via the TOC/TIC translocon complexes and the physical division of the organelle. While these processes are mechanistically distinct, their evolutionary integration remains unclear. Specifically:

• How did the host cell gain control over the division of the endosymbiotic cyanobacterium?
• How is the import machinery (TOC/TIC) coordinated with the division machinery (FTZ/ARC6)?
• What is the evolutionary origin of the TOC GTPases, which are essential for protein import but whose ancestry has been a mystery?
• Septins are GTP-binding proteins known to regulate membrane curvature and division in animals and fungi (Opisthokonts), but their function and evolutionary history in non-Opisthokont lineages (like algae) are poorly understood.

2. Methodology

The authors employed a multidisciplinary approach combining genetics, cell biology, structural biology, and phylogenetics using the green alga Chlamydomonas reinhardtii as the primary model, with validation in other algae and land plants.

• Genetic Manipulation: CRISPR-Cas9 was used to generate endogenous tags (SEP1-NG-3FLAG), knockouts (sep1Δ), and double mutants (sep1Δ nap1-1) to study septin function.
• Imaging:
  • Live-cell imaging: Time-lapse microscopy to track cell cycle progression and organelle dynamics.
  • Ultrastructure Expansion Microscopy (U-ExM): Used to physically expand cells 4-fold to achieve ~44 nm resolution, allowing visualization of filamentous structures on the chloroplast envelope.
  • Super-resolution microscopy (SRRF/Airyscan): To analyze colocalization of SEP1 with TOC components and division rings.
• Proteomics & Interaction Studies:
  • Immunoprecipitation Mass Spectrometry (IP-MS): To identify SEP1 interactors.
  • BioID (Proximity Labeling): To map transient interactions between SEP1 and chloroplast-targeted proteins.
  • Yeast Two-Hybrid (Y2H): To test direct interactions between GTPase domains of SEP1 and TOC receptors.
• Functional Assays:
  • Latrunculin B (LatB) Sensitivity: Used to perturb the actin cytoskeleton and reveal synthetic defects in sep1Δ mutants.
  • Import Efficiency Assays: Measured the accumulation of chloroplast-targeted reporter proteins (fused to mScarlet) in WT vs. sep1Δ backgrounds, specifically testing division proteins (FTSZ, ARC6) vs. constitutive proteins (RBCS2).
• Phylogenetics: Maximum-likelihood analysis of GTPase domains across diverse eukaryotes (Opisthokonts, Archaeplastida, Chromista) to trace the evolutionary origin of TOC GTPases.
• Heterologous Expression: Expression of Chlamydomonas SEP1 in Volvox carteri, Physcomitrium patens (moss), Arabidopsis thaliana, and Nicotiana benthamiana to test conservation of function in lineages that have lost native septins.

3. Key Contributions & Results

A. SEP1 Localization and Structure

• Filament Formation: In C. reinhardtii, the single septin gene SEP1 forms a filamentous network on the chloroplast outer envelope during interphase.
• Ring Assembly: During cytokinesis, these filaments reorganize into a distinct ring at the chloroplast division site (midzone), persisting through cell division.
• Curvature Sensing: The C-terminal amphipathic helix (AH) of SEP1 is required for membrane association. SEP1 filaments sense micron-scale membrane curvatures, similar to yeast septin complexes.

B. SEP1 Regulates Chloroplast Division and Protein Import

• Synthetic Phenotype: sep1Δ mutants show no severe defects under standard conditions. However, when combined with an actin-deficient background (nap1-1) and treated with LatB, sep1Δ cells exhibit severe chloroplast division defects (lobed, mispositioned chloroplasts) and delayed fission.
• Mispositioning of Division Machinery: In sep1Δ mutants treated with LatB, the division proteins FTSZ1, FTSZ2, and ARC6 are mispositioned and fail to form a focused ring at the chloroplast midzone.
• Selective Import Defect: Loss of SEP1 specifically impairs the import of chloroplast-division proteins (FTSZ1, FTSZ2, ARC6) but does not affect the import of constitutive proteins like RBCS2. This suggests SEP1 regulates substrate selectivity or efficiency for specific preproteins.

C. Physical Interaction with TOC Translocon

• Direct Binding: SEP1 physically interacts with the outer envelope TOC GTPases, specifically TOC90 and TOC120 (TOC159-family), but not TOC34.
• G-Interface Interaction: Yeast two-hybrid and structural modeling (FragFold) indicate that SEP1 binds TOC90 via their conserved GTPase (G) domains.
• Temporal Dynamics: SEP1 and TOC90 colocalize on the chloroplast envelope during the G1 phase of the cell cycle. However, as the cell enters mitosis, TOC90 is removed from the division site while SEP1 remains to form the division ring, suggesting SEP1 has dual roles: facilitating import in G1 and organizing division in M-phase.

D. Evolutionary Insights

• TOC GTPases are Septin-Derived: Phylogenetic analysis reveals that TOC GTPases form a monophyletic clade branching within the septin family, specifically clustering with septins from early-diverging algae. This suggests TOC receptors evolved from an ancestral algal septin.
• Conservation Across Evolution: Despite the loss of septins in land plants, heterologous expression of Chlamydomonas SEP1 in moss, Arabidopsis, and tobacco results in:
  • Targeting to the chloroplast envelope.
  • Formation of filaments and rings.
  • Physical interaction with land-plant TOC GTPases.
  • This indicates the functional link between septins and the chloroplast translocon was established early in Archaeplastida evolution and has been retained for over 700 million years.

4. Significance

• Mechanistic Link: The study identifies a novel molecular mechanism where a septin (SEP1) acts as a coordinator between protein import and organelle division, ensuring that division proteins are efficiently imported and correctly positioned.
• Evolutionary Origin of TOC: It resolves the long-standing mystery of the TOC GTPase origin, proposing that they are derived from an ancestral septin that adapted to recognize the cyanobacterial endosymbiont and facilitate protein import.
• Ancestral Septin Function: The findings suggest that the ancestral function of septins (present in the Last Eukaryotic Common Ancestor, LECA) involved binding to bacterial surfaces and regulating their fission. This function was co-opted during primary endosymbiosis to manage the nascent chloroplast.
• Paradigm Shift: It challenges the view of the chloroplast import machinery as a static channel, showing it is dynamically regulated by septins to couple biogenesis with cell division.

In summary, the paper demonstrates that septins are ancient, conserved regulators of plastid biogenesis that physically and functionally couple the import of division proteins with the spatial organization of the chloroplast division machinery, a mechanism rooted in the early evolutionary history of photosynthetic eukaryotes.