At least two mobile genetic elements can be observed in certain strains of Bacillus subtilis – the integrative and conjugative element ICEBs1 [1,2], and the temperate phage SPß [3]. The spbK gene in ICEBs1 inhibits the production of active SPß. Its gene product, BsSPBK1, contains a TIR domain that is necessary for function [4]. TIR domains are known to show NADase activity [5], suggesting that BsSPBK1 could also be an NADase effector and elicits abortive infection that leads to cell death. The abortive infection depends on the SPß gene yonE, where co-expression of spbK and yonE inhibits the growth of host cells [4]. The exact function of yonE is not known yet. Its encoded protein, YonE, shares similarity with viral capsid portal proteins [6]. The mechanism of how YonE and BsSPBK1 trigger an antiphage response is poorly understood. In this study, we have confirmed that BsSPBK1 has NADase activity and is modulated by YonE. A ~3Å cryoEM reconstruction of YonE suggests that the protein assembles into a dodecamer as expected for portal proteins. The EM density map fits well with the YonE dodecameric model predicted by AlphaFold2 [7]. We have also obtained a ~3.3Å cryoEM reconstruction of BsSPBK1 filaments and model fitting suggests that the TIR domains self-assemble to form filaments. We hypothesize that this self-assembly of TIR domains in BsSPBK1 forms active sites capable of cleaving NAD+ [8]. Negative stain electron microscopy results have indicated that filament formation is enhanced in the presence of YonE, resulting in a higher density of longer filaments. A cryoEM structure of the YonE- BsSPBK1 complex will provide key insights into how YonE interacts with BsSPBK1 to modulate its NADase activity. To this end, we have produced and screened a mutant of BsSPBK1 that forms shorter filaments with less bundling, and are now in the process of collecting cryoEM data in the presence of YonE.