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 His-Xaa-Xaa-Ile-Leu-Leu-Thr), which allows it to activate CdnC for second messenger synthesis. Protects E.coli strain JP313 against bacteriophage lambda cI- infection. When the cdnC-cap7-cap6-nucC operon is transformed into a susceptible strain it confers bacteriophage immunity. Mutations in the sensor (Cap7 also called HORMA) or effector proteins (CdnC, NucC) but not the disassembly protein (Cap6 also called Trip13) no longer confer immunity. The presence of the intact operon leads to culture collapse and cell death, which occurs before the phage has finished its replication cycle, thus protecting non-infected bacteria by aborting the phage infection and preventing its propagation. Forms complexes with CdnC with 1:1 and 2:2 stoichimetry, and a 1:1:6 CdnC:Cap7:Cap6 complex. In the disassembly complex (PDB:6P8V) Cap6 (also called Trip13) crystallizes as a right-handed spiral; the top 4 subunits bind ATP while the bottom 2 do not. A CdnC monomer lies along the surface of the hexamer at the interface of subunits 5 and 6, with Cap7 (this protein) at its tip, over the central hexamer pore. The N-terminus of Cap7 extends into the pore, contacting 5/6 Cap6 subunits. Belongs to the bacterial HORMA family. HORMA1 subfamily. Truncated N-terminus.