CBASS (cyclic oligonucleotide-based antiphage signaling system) provides immunity against bacteriophage. The CD-NTase protein synthesizes cyclic nucleotides in response to infection; these serve as specific second messenger signals. The signals activate a diverse range of effectors, leading to bacterial cell death and thus abortive phage infection. A type III-C(AAA) CBASS system (PubMed:32839535). The sensor protein for this CBASS system. Binds to a closure peptide (consensus Glu-Val-Met-Glu-Phe-Asn-Pro), which allows it to activate CdnD for second messenger synthesis. Interacts individually with CdnD and Cap8 (also called HORMA3); forms CdnD:Cap7:Cap8 (also called CdnD:HORMA2:HORMA3) complexes with stoichiometries of 1:1:1 and 2:1:1. Can adopt both open and closed states. Full-length closed state protein binds a closure peptide (consensus Glu-Val-Met-Glu-Phe-Asn-Pro). In the Cap7:Cap8 complex only Cap7 binds a closure peptide. Belongs to the bacterial HORMA family. HORMA2 subfamily.