The Raf/LIN-45 C-terminal distal tail segment negatively regulates signaling inCaenorhabditis elegans

Abstract

AbstractRaf protein kinases act as Ras-GTP sensing components of the ERK signal transduction pathway in animal cells, influencing cell proliferation, differentiation, and survival. In humans, somatic and germline mutations in the genesBRAFandRAF1are associated with malignancies and developmental disorders. Recent studies shed light on the structure of activated Raf, a heterotetramer consisting of Raf and 14-3-3 dimers, and raised the possibility that a Raf C-terminal distal tail segment (DTS) regulates activation. We investigated the role of the DTS using theCaenorhabditis elegans,which has a single Raf ortholog termedlin-45. We discovered that truncations removing the DTS strongly enhancedlin-45(S312A), a weak gain-of-function allele equivalent toRAF1mutations found in patients with Noonan Syndrome. We generated mutations to test three elements of the LIN-45 DTS, which we termed the active site binding sequence (ASBS), the KTP motif, and the aromatic cluster. In the context oflin-45(S312A),mutation of either the ASBS, KTP motif, or aromatic cluster enhanced activity. We used AlphaFold to predict DTS protein interactions for LIN-45, fly Raf, and human BRAF, within the activated heterotetramer complex. We propose distinct functions for the LIN-45 DTS elements: i) the ASBS binds the kinase active site as an inhibitor, ii) phosphorylation of the KTP motif modulates DTS-kinase domain interaction, and iii) the aromatic cluster anchors the DTS in an inhibitory conformation. This work establishes that the Raf/LIN-45 DTS negatively regulates signaling inC. elegansand provides a model for its function in other Raf proteins.