Saturday, June 12, 2010

Ubiquitination Unplugged

Of various post-translational modifications, multi-faceted roles of Ubiquitination are being uncovered in recent years. Ubiquitination had once became synonymous with proteasomal degradation in eukaryotes. But two discoveries marked departure from this notion. One was the identification of Prokaryotic Ubiquitin-like protein (Pup) and the equivalent process termed Pupylation. Second discovery was that Ubiquitin conjugation can indeed modulate cellular signalling apart from proteasomal degradation.

In case of peroxisomes, Ubiquitination process is exploited by the cell to exert quality control as well as recycling of some peroxins for import cycles. Pex5 was the first peroxin shown to be mono or polyubiquitinated for different purposes. Pex4 mediated monoubiquitination marks Pex5 for recycling the receptor for next round of import. Whereas if the Pex5 is no longer functional or has stuck in the membrane, Ubc4 dependant polyubiquitination primes Pex5 for proteasomal degradation. (Ref.)
Pex7 is the cognate receptor for PTS2 containing proteins but requires auxiliary proteins(co-receptors) Pex18, Pex21 or Pex20 in yeasts. These co-receptors are functionally equivalent to N-terminal part of Pex5 involved in peroxisomal docking and recycling. Pex18 is constitutively degraded via polyubiquitination and seems not to be recycled unlike Pex5. Pex20 is also polyubitinated and signals the degradation.

What are common misconceptions

Monoubiquitination means one Ubiquitin?
Yes, but Pex5 can be ubiquitinated at more than one amino acid residue using single Ubiquitin moiety. While polyubiquitination indicates attachment of Ubiquitin chain of at least 4 Ub molecules.

Lysine residues in a protein are the only site for Ubiquitination?
Not in all cases. At least for mammalian Pex5, unconventionally Cysteine residue is also utilized for ubiquitination.

Polyubiquitination is the signal only for proteasomal degradation?
K48-linked polyUb chains do signal for proteasomal degradation. But Ubiquitin has seven lysine residues and in a stunning observation made by Peng J. et al that all Lysine residues can be utilized for this “Atypical” chain formation and K63-linked Poly-Ub chains are emerging as regulator of diverse processes unlike proteasomal degradation via K48-chains

Recent CELL paper by Zeng et al. titled “Reconstitution of the RIG-I Pathway Reveals a Signaling Role of Unanchored Polyubiquitin Chains in Innate Immunity”, makes staggering discoveries.
First is that, K63-linked polyubiquitin chains have important functional role. And most interestingly, that these chains need not be conjugated to a protein, but can associate through protein interaction domains. The authors show for the first time that free poly-Ubiquitin chains in the cytosol can directly associate with a cytosolic protein without being conjugated by isopeptide bond and modulate cell signalling. The Unanchored K63-ubiquitin chains in the cytosol were found to activate RIG-I which detects RNA viruses and signals antiviral response through MAVS.

CAspase Recruitment Domain (CARD) is the hallmark feature of RIG-I family members and MAVS. CARD domains of RIG-I interact with unanchored K63-Ub chains leading to RIG-I activation. The activated RIG-I in turn interacts with Mitochondrial MAVS leading to activation of downstream targets. It was also demonstrated by the authors that CARD domain of MAVS is not ubiquitinated unlike RIG-I.

Though this discovery is of general scientific interest,
Why it would be interesting for peroxisome researchers?

In another recent paper in CELL by Dixit et al. titled “Peroxisomes Are Signaling Platforms for Antiviral Innate Immunity”, it was unequivocally established that MAVS resides not only on mitochondria but also on peroxisomes. Surprisingly peroxisome localized MAVS elicits rapid but transient antiviral response in the form of activation of ISG (Interferon Stimulated Genes), albeit independent of type I-interferon. This downstream cascade differs from that of MAVS localized on mitochondria!!! How same protein (MAVS) residing on two different organelles induce entirely different downstream signalling cascade remains unknown.

I propose some possible hypothetical explanations

- It will be interesting to know whether peroxisome localized MAVS also requires activation of RIG-I by K63-branched PolyUb or not. It might be possible that viral RNA-RIG complex could somehow directly activate MAVS on peroxisomes. Whereas sustained Mitochondrial MAVS might require stable association of RIG-I and K63-Ub chains.

- Possibly other peroxisomal components are involved. maybe peroxisomal membrane proteins?
Trans-membrane domain of MAVS is essential for its self-association as well as functionality. But It could be replaced by TM from other membrane proteins such as Bcl-xl. This indicates that TM domain of MAVS can interact not only with itself but with TM of other membrane proteins. And this fact can give rise to differences in downstream cascade.

- Does Peroxisome motility an important factor determining rapid response by peroxisomal MAVS?
Peroxisomes exhibit dynamic cytoskeleton dependant movement within the cytoplasm. Mitochondria also show such motility but peroxisomes are faster..!

The information from above two CELL papers can be summarized in this representative figure. 
Some things remain unknown (Question mark) 

Coming back to fascinating ubiquitins, there is an excellent review in SCIENCE titled “Anchors Away for Ubiquitin Chains” which discusses the Zeng et al paper and speculates whether the Unanchored polyUb chains are involved in more pathways other than RIG-I-MAVS in innate immunity… (And in case of our interest, in peroxisome biogenesis!).

To close the topic on a high note, It must be noted that K11-linked Ub chains are as abundant as K63 ones in S.cereviseae, yet their function is not even remotely known. And interestingly even branched PolyUb chains containing multiple K-linkages in a single chain have also been discovered….

Thus the complexity heightens so does the Curiosity!!

1 comment:

  1. An interesting Paper has appeared in Cell Research
    "Tom70 mediates activation of interferon regulatory factor 3 on mitochondria"
    The paper establishes that TOM70, the mitochondrial import receptor recruits MAVS upon RNA virus infection. Does any peroxisome import receptor is involved in peroxisomal MAVS recruitment?