The STE20 kinases MST1 and MST2 are key players in mammalian Hippo pathway. The SARAH domains of MST1/2 act as a platform to mediate homodimerization and hetero-interaction with a range of adaptors including RASSFs and Salvador, which also possess SARAH domains. Here, we determined the crystal structure of human MST2 SARAH domain, which forms an antiparallel homodimeric coiled coil. Structural comparison indicates that SARAH domains of different proteins may utilize a shared dimerization module to form homodimer or heterodimer. Structure-guided mutational study identified specific interface residues critical for MST2 homodimerization. MST2 mutations disrupting its homodimerization also impaired its hetero-interaction with RAPL (also named RASSF5 and NORE1), which is mediated by their SARAH domains. Further biochemical and cellular assays indicated that SARAH domain-mediated homodimerization and hetero-interaction with RAPL are required for full activation of MST2 and therefore apoptotic functions in T cells. (C) 2014 Elsevier Inc. All rights reserved.