Second messenger Ap4A polymerizes target protein HINT1 to transduce signals in FcεRI-activated mast cells

Abstract

Signal transduction systems enable organisms to monitor their external environments and accordingly adjust the cellular processes. In mast cells, the second messenger Ap4A binds to the histidine triad nucleotide-binding protein 1 (HINT1), disrupts its interaction with the microphthalmia-associated transcription factor (MITF), and eventually activates the transcription of genes downstream of MITF in response to immunostimulation. How the HINT1 protein recognizes and is regulated by Ap4A remain unclear. Here, using eight crystal structures, biochemical experiments, negative stain electron microscopy, and cellular experiments, we report that Ap4A specifically polymerizes HINT1 in solution and in activated rat basophilic leukemia cells. The polymerization interface overlaps with the area on HINT1 for MITF interaction, suggesting a possible competitive mechanism to release MITF for transcriptional activation. The mechanism depends precisely on the length of the phosphodiester linkage of Ap4A. These results highlight a direct polymerization signaling mechanism by the second messenger.

Re-evaluation of Diadenosine Tetraphosphate (Ap4A) From a Stress Metabolite to Bona Fide Secondary Messenger

“…Ap 4 A bridges the HINT1 dimer-dimer interface promoting association with the adjacent dimer facilitating their polymerization (Yu et al, 2019). Following HINT1 polymer formation, MITF translocates to the nucleus to promote transcription of target genes and initiate an allergic response (6) (Yu et al, 2019). proximal to the Ap 4 A binding site in HINT1, suggesting a competitive mechanism between Ap 4 A and MITF for HINT1 binding (Figure 3; Yu et al, 2019).…”

“…Following HINT1 polymer formation, MITF translocates to the nucleus to promote transcription of target genes and initiate an allergic response (6) (Yu et al, 2019). proximal to the Ap 4 A binding site in HINT1, suggesting a competitive mechanism between Ap 4 A and MITF for HINT1 binding (Figure 3; Yu et al, 2019). The specificity of this transcriptional response for a dinucleoside tetraphosphate arises from the specific requirements for oligophosphate chain length for the oligomerization of HINT1.…”

“…Of the dinucleotides tested only Ap 4 A has so far been found to inhibit DNA replication. The effect of Ap 4 A in the HINT1-MITF (Yu et al, 2019) and cGAS-STING (Guerra et al, 2020) FIGURE 5 | Ap 4 A-mediated inhibition of DNA replication model. The first step of DNA replication involves the formation of the Pre-replication complex (1), where Cdc6 and the origin recognition complex (ORC) promote formation of the Mcm2-7 double hexamer through the recruitment of Cdt1 molecules (Takara and Bell, 2011).…”

“…Ap 4 A binding to HINT1 promotes its polymerization (5) . Ap 4 A bridges the HINT1 dimer–dimer interface promoting association with the adjacent dimer facilitating their polymerization ( Yu et al, 2019 ). Following HINT1 polymer formation, MITF translocates to the nucleus to promote transcription of target genes and initiate an allergic response (6) ( Yu et al, 2019 ).…”

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“…Sin embargo, un estudio en el que se examinan cuidadosamente una serie de criterios para ser calificado como tal parece demostrar que este no es el caso en Escherichia coli (Despotović et al 2017). En contraste, recientemente se ha sugerido su posible papel como regulador en mastocitos (Yu et al 2019). Por ello la idea general de reclutar un subproducto para una función sigue siendo atractiva ya que los caminos de la evolución son impredecibles.…”