NBR1 shuttles between the cytoplasm and nucleus and is essential for nuclear p62 body formation

This study demonstrates that NBR1, like its partner p62, shuttles between the cytoplasm and nucleus via specific signal motifs and is essential for the formation of nuclear p62 bodies.

The Gist

The Big Picture: The Cell's Cleanup Crew

Imagine your cell is a bustling, high-tech city. Like any city, it produces trash (damaged proteins) and needs a way to get rid of it. The cell has a specialized "cleanup crew" called autophagy.

Two of the most important workers on this crew are proteins named p62 and NBR1.

• p62 is like the foreman. It's the main organizer that grabs onto the trash (which is often marked with a "sticky note" called ubiquitin) and builds a giant pile called a p62 body. This pile is then sent to the cell's recycling plant (the lysosome) to be destroyed.
• NBR1 is the assistant foreman. It helps p62 do its job, but until now, scientists weren't sure exactly what it was doing or where it was working.

The Discovery: NBR1 Has a Commute

For a long time, scientists thought NBR1 only worked in the "streets" of the cell (the cytoplasm). But this paper reveals a surprising secret: NBR1 also has a job inside the "city hall" (the nucleus).

The nucleus is the control center where the cell's DNA is kept. It's a very sensitive area. If trash builds up there, it can cause serious problems, like cancer or neurodegenerative diseases.

The researchers found that NBR1 is a commuter. It travels back and forth between the streets and city hall.

• The Exit Pass (NES): NBR1 has a special "exit pass" (a signal called NES) that tells the city's security guards to let it leave the nucleus and go back to the streets.
• The Entry Pass (NLS): It also has an "entry pass" (a signal called NLS) that lets it get back into city hall.

The researchers discovered that NBR1 has two exit passes and one entry pass. If you block the exit passes (using a drug called Leptomycin B), NBR1 gets stuck inside the nucleus, and the city hall fills up with NBR1.

The Teamwork: You Can't Build a Pile Without the Assistant

The most important finding of this paper is about how p62 and NBR1 work together, specifically inside the nucleus.

Think of building a p62 body like building a sandcastle.

• p62 is the sand. It can pile up on its own, but it's messy and unstable.
• NBR1 is the mold or the structure that holds the sand together.

The paper shows that:

1. NBR1 is essential: If you remove NBR1 (like taking away the mold), the sand (p62) cannot form a proper castle (p62 body) inside the nucleus. The cleanup crew fails to organize the trash.
2. They need each other: NBR1 helps p62 grab onto the trash. Without NBR1, p62 is like a foreman trying to organize a pile of garbage with no hands.
3. The "Liquid" Nature: These p62 bodies aren't solid rocks; they are more like liquid droplets (think of a drop of water merging with another). The researchers showed that these droplets can flow and merge, but NBR1 makes this process much faster and more efficient.

Why Does This Matter?

Imagine a specific type of trash in the city hall: Ataxin-1. This is a mutant protein that causes a serious brain disease called Spinocerebellar Ataxia.

The researchers found that when this toxic trash appears in the nucleus:

• With NBR1: The cleanup crew (p62 and NBR1) rushes in, forms a liquid droplet around the trash, and brings in the "garbage disposal unit" (the proteasome) to destroy it.
• Without NBR1: The cleanup crew is confused. p62 shows up, but it can't form a proper pile. The trash stays there, clogging up the city hall, which leads to cell damage and disease.

The Takeaway

This paper tells us that NBR1 is not just a helper; it is a critical manager.

• It shuttles between the cell's main office and the control center.
• It is absolutely required to build the "trash piles" (p62 bodies) inside the nucleus.
• Without NBR1, the cell cannot effectively clean up toxic protein clumps in the nucleus, which could be a key factor in diseases like Huntington's or Alzheimer's.

In short: NBR1 is the glue that holds the cell's nuclear cleanup crew together. Without it, the trash piles up, and the city (the cell) starts to fall apart.

Technical Summary

1. Problem Statement

Selective autophagy receptors SQSTM1/p62 and NBR1 are critical for the degradation of protein aggregates and organelles via autophagy. They function by forming cytoplasmic condensates known as p62 bodies, which rely on the interaction between polymeric p62 and polyubiquitinated substrates. While p62 is known to shuttle between the cytoplasm and nucleus (facilitating nuclear protein quality control), the nuclear dynamics of NBR1 were unknown. Specifically, it was unclear whether:

• NBR1 shuttles between the nucleus and cytoplasm.
• NBR1 is a component of nuclear p62 bodies.
• NBR1 is required for the formation of nuclear p62 bodies.

2. Methodology

The authors employed a combination of cell biology, molecular genetics, and live-cell imaging techniques using human cell lines (HEK293, HeLa, U2OS):

• Nuclear Export/Import Analysis: Cells were treated with Leptomycin B (LMB), an inhibitor of the nuclear export receptor CRM1, to observe the accumulation of endogenous and exogenous NBR1 in the nucleus.
• CRISPR/Cas9 Knockout (KO): Generation of p62 KO, NBR1 KO, and double KO (DKO) HeLa cell lines to assess the dependency of nuclear localization and body formation on each protein.
• Domain Mapping & Mutagenesis:
  • NES Identification: Bioinformatic prediction (LocNES) followed by the creation of deletion mutants (ΔNES1, ΔNES2, ΔNES1/2) and reporter assays using the pRev(1.4)-EGFP system to validate Nuclear Export Signals.
  • NLS Identification: Systematic deletion of NBR1 domains (including the Coiled-Coil 1 or CC1 domain) and fusion to EGFP-β-galactosidase (a large protein that cannot diffuse into the nucleus) to identify Nuclear Localization Signals.
• Immunofluorescence & Co-localization: Staining for p62, NBR1, PML bodies, ubiquitinated proteins (FK2), and proteasomal subunits (β2) to determine spatial relationships.
• Protein Aggregation Models: Transfection of EGFP-Ataxin-1 (Atx84Q), a polyglutamine-expanded protein that forms nuclear inclusions, to test recruitment of p62/NBR1 and proteasomes.
• Biophysical Characterization:
  • FRAP (Fluorescence Recovery After Photobleaching): To test the liquid-like properties of nuclear condensates.
  • 1,6-Hexanediol Treatment: To disrupt weak hydrophobic interactions characteristic of liquid-liquid phase separation (LLPS).

3. Key Contributions & Results

A. NBR1 Shuttles Between Cytoplasm and Nucleus

• NES Discovery: NBR1 contains two functional Nuclear Export Signals: NES1 (residues 169–185) and NES2 (residues 557–572). NES1 is the dominant exporter, but both contribute to maintaining cytoplasmic localization.
• NLS Discovery: NBR1 contains a functional Nuclear Localization Signal within its Coiled-Coil 1 (CC1) domain (specifically residues 309–310, K309/K310). This is the first identification of an NLS within a coiled-coil domain of an autophagy receptor.
• Shuttling Mechanism: Endogenous NBR1 accumulates in the nucleus upon LMB treatment, confirming active shuttling. However, overexpression of NBR1 leads to cytoplasmic aggregation (via the CC1 domain), which sequesters the protein and reduces nuclear import efficiency.

B. NBR1 is Essential for Nuclear p62 Body Formation

• Co-localization: Nuclear NBR1 puncta co-localize perfectly with p62 and ubiquitinated proteins, and partially with PML bodies.
• Dependency:
  • In p62 KO cells, nuclear NBR1 puncta formation is severely reduced, indicating NBR1 relies on p62 to form stable nuclear condensates.
  • In NBR1 KO cells, the formation of nuclear p62 bodies is significantly impaired, even when nuclear export is blocked by LMB.
• Interaction Requirement: The direct interaction between NBR1 and p62 via their PB1 domains is mandatory. A D50R mutation in NBR1 (disrupting PB1 interaction) prevents NBR1 from facilitating p62 nuclear accumulation, even if NBR1 itself accumulates in the nucleus.

C. Role in Nuclear Protein Quality Control

• Recruitment to Inclusions: In cells expressing mutant Ataxin-1 (Atx84Q), both p62 and NBR1 are recruited to the surface of nuclear inclusions. This recruitment is mutually dependent; loss of either protein significantly reduces the recruitment of the other.
• Proteasome Recruitment: p62 and NBR1 co-recruit the 26S proteasome (subunit β2) to these nuclear inclusions. In double KO cells, proteasome recruitment is abolished.
• Liquid Phase Properties: Nuclear p62 bodies exhibit liquid-like dynamics (rapid FRAP recovery). The presence of NBR1 accelerates the recovery rate of p62 condensates, suggesting NBR1 facilitates the efficient formation and fluidity of these nuclear bodies. Both p62 and NBR1 condensates are sensitive to 1,6-hexanediol.

4. Significance

• Mechanistic Insight: The study establishes that NBR1 is not merely a cytoplasmic partner of p62 but an active participant in nuclear protein quality control.
• Essentiality: It demonstrates that NBR1 is essential for the formation of nuclear p62 bodies, acting as a critical facilitator that likely regulates filament length or recruits substrates/proteasomes more efficiently than p62 alone.
• New Signaling Motifs: The identification of an NLS within the CC1 domain and two NES motifs expands the understanding of how autophagy receptors are regulated by nucleocytoplasmic transport.
• Pathological Relevance: Given the link between nuclear protein aggregates (like those in Huntington's or Spinocerebellar Ataxia) and neurodegeneration, these findings suggest that the NBR1-p62 axis is a critical defense mechanism in the nucleus. Disruption of this shuttle or the NBR1-p62 interaction could impair the degradation of toxic nuclear aggregates.

Conclusion

The paper concludes that NBR1 shuttles between the nucleus and cytoplasm via specific NLS and NES motifs. Crucially, NBR1 is required for the efficient formation of nuclear p62 bodies, where it acts in concert with p62 to recruit proteasomes and degrade ubiquitinated nuclear substrates. This highlights a sophisticated, interdependent mechanism for nuclear protein homeostasis.