Tight regulation of immune responses is important for overall fitness, but the mechanisms underlying many of these pathways are incompletely characterized. One example is the recently described "Intracellular Pathogen Response" (IPR) in C. elegans. Our lab has defined the IPR as a set of ~80 genes that are upregulated in response to several triggers including intracellular pathogen infection, proteasome inhibition and heat stress. The IPR is also genetically regulated by the species-specific antagonistic paralogs pals-22 and pals-25. pals-22 represses the IPR; loss-of-function mutations in pals-22 promote IPR gene expression, increase pathogen resistance and improve tolerance of proteotoxic stress. pals-25 acts downstream of pals-22 to activate the IPR; loss-of-function mutations in pals-25 suppress the phenotypes of pals-22 mutants. Recently, we identified a gain-of-function mutation of pals-25 that truncates the C-terminus of PALS-25 by 13 amino acids, or ~5% of the total protein. Unlike previously characterized mutations of pals-25, this pals-25(gf) allele results in constitutive expression of IPR genes in both wild type and pals-22 mutant backgrounds. However, only a subset of IPR genes appear to be upregulated in pals-25(gf) mutants when compared to pals-22 mutants. pals-25(gf) animals are similar to pals-22 mutants in that they are resistant to infection by Nematocida parisii but are dissimilar in that they display wild type tolerance of heat stress. Together, these observations may allow for the identification of IPR-related genes that are specifically important for pathogen resistance phenotypes. Previous co-IP/MS studies determined that PALS-22 and PALS-25 are physically associated. Here we show that FLAG-IP of PALS-22::GFP::3xFLAG identifies PALS-25 as a binding partner, but this interaction is no longer detected for the C-terminally truncated version of PALS-25 encoded by the pals-25(gf) allele. Yeast two-hybrid analysis of full-length PALS-25 also revealed PALS-22 as a binding partner, but the truncated version of PALS-25 did not identify PALS-22 as a binding partner. Together, our results suggest a model where PALS-22 physically represses the ability of PALS-25 to activate the IPR and the interaction of the two proteins requires the C-terminus of PALS-25. Ongoing studies will explore the mechanism of IPR activation by PALS-25 after it is released from repression by PALS-22.

Affiliations:
- Department of Life Sciences, Imperial College, London, United Kingdom
- Molecular Neurobiology Laboratory, Salk Institute for Biological Studies, La Jolla, CA
- Division of Biological Sciences, University of California, San Diego, La Jolla, CA