14-3-3 and erlin proteins differentially interact with RIPK2 complexes.

The receptor interacting serine/threonine kinase 2 (RIPK2) is essential for signal transduction induced by the pattern recognition receptors NOD1 and NOD2 (referred to collectively as NOD1/2). Upon NOD1/2 activation, RIPK2 forms complexes in the cytoplasm of human cells. Here, we identified the molecular composition of these complexes. Infection with Shigella flexneri to activate NOD1-RIPK2 revealed that RIPK2 formed dynamic interactions with several cellular proteins, including A20 (also known as TNFAIP3), erlin-1, erlin-2 and 14-3-3. Whereas interaction of RIPK2 with 14-3-3 proteins was strongly reduced upon infection with Shigella, erlin-1 and erlin-2 (erlin-1/2) specifically bound to RIPK2 complexes. The interaction of these proteins with RIPK2 was validated using protein binding assays and immunofluorescence staining. Beside bacterial activation of NOD1/2, depletion of the E3 ubiquitin ligase XIAP and treatment with RIPK2 inhibitors also led to the formation of RIPK2 cytosolic complexes. Although erlin-1/2 were recruited to RIPK2 complexes following XIAP inhibition, these proteins did not associate with RIPK2 structures induced by RIPK2 inhibitors. While the specific recruitment of erlin-1/2 to RIPK2 suggests a role in innate immune signaling, the biological response regulated by the erlin-1/2-RIPK2 association remains to be determined.