AfCS-Nature Signaling Gateway - signaling update

signaling update home

updates

news

research library

CELL SIGNALLING: PYRIN fever

SOURCE:

home | subscribe

A new member of the PYRIN-domain family has been identified as a novel activator of the NF?B signalling pathway.

The PYRIN domain is a recently discovered protein–protein interaction motif found at the amino termini of several proteins involved in apoptotic and inflammatory signalling pathways. Analysis of the human genome predicts a large family of PYRIN-containing proteins, some of which are thought to interact through their PYRIN domains. A report in the Journal of Biological Chemistry now describes a new member of the PYRIN-domain family, and identifies it as a novel activator of the NF- kappa B signalling pathway.

John Bertin and colleagues at Millennium Pharmaceuticals searched an expressed-sequence-tag database to identify members of the nucleotide-binding site/leucine-rich repeat (NBS/LRR) family that have an amino-terminal PYRIN domain. They pulled out PYPAF1 ('PYRIN-containing Apaf1-like protein 1'), which also contains an NBS domain and a number of carboxy-terminal LRRs. The authors then carried out a two-hybrid screen and found that PYPAF1 interacts selectively with the PYRIN domain of a protein called ASC — an interaction that requires the PYRIN domain of PYPAF1.

Bertin's group next carried out cellular co-localization studies of the two proteins. Whereas PYPAF1 alone showed a broad cytoplasmic distribution, co-expression with ASC led to PYPAF1 being recruited to punctate structures in the cytoplasm. The PYRIN domain of PYPAF1 was needed for this recruitment, supporting the idea that this domain mediates the assembly of a PYPAF1/ASC complex.

But what does this complex do? Northern blotting analysis revealed that PYPAF1 is expressed mainly in peripheral blood leukocytes, suggesting a possible role in inflammatory signalling. Moreover, other NBS/LRR proteins are known to act as upstream regulators of NF- kappa B signalling. So Bertin and colleagues studied the activation of NF- kappa B using a luciferase reporter plasmid.

Whereas PYPAF1 alone did not induce the activation of NF- kappa B, expression of ASC alone could — but only at high concentrations. However, co-expression of low levels of ASC (which did not activate NF- kappa B) with PYPAF1 led to a 30-fold increase in NF- kappa B activity. Deletion of PYPAF1's PYRIN domain blocked this stimulation. Surprisingly, deletion of a carboxy-terminal region resulted in a twofold increase in the synergistic activation, and the authors suggest that the LRRs could act as a negative regulator of PYPAF1 activity. Finally, they showed that the NF- kappa B signalling occurs through the IKK complex, as dominant-negative versions of IKK- gamma and IKK-2 blocked the synergistic activation of NF- kappa B.

The emerging story, then, is that PYRIN-containing proteins are important in innate immunity. Defects in the archetypal family member, pyrin, are associated with a rare inflammatory disease called familial Mediterranean fever. And as pyrin binds to ASC, the authors speculate that their findings point to a link between pyrin and the NF- kappa B signalling pathway.

Alison Mitchell

References

ORIGINAL RESEARCH PAPER
Manji, G. A. et al. PYPAF1: A PYRIN-containing Apaf1-like protein that assembles with ASC and regulates activation of NF-B. J. Biol. Chem. 10 January 2002 (DOI 10.1074/jbc.M112208200)