Receptor expressed in lymphoid tissues (BELT) is a new member of the TNFR family with little known regarding its signaling. Typically, TNFRs engage TRAFs for activation of NF-kappa B and MAPK cascades. We found that RELT does not use the standard signaling pathways characteristic of other TNFRs. While overexpression of RELT in 293 cells induced p38 and JNK activation, it did not activate NF-kappa B. In addition, no binding oh RELT to TRAF1,2,3,5, or 6 was detected. Using a yeast two-hybrid system, we identified a Ste20-related proline-alanine-rich kinase (SPAK) that binds RELT. Disruption of the SPAK binding motif, (RFRV)-R-349, in RELT inhibited RELT activation of p38 and JNK. In addition, a kinase-dead SPAK acted as an inhibitor of RELT signaling. Thus, we conclude that RELT does not rely on the canonical TRAF pathways For its function, but instead uses a kinase, SPAK, to mediate p38 and JNK activation. (c) 2006 Elsevier Inc. All rights reserved.