SpoIVB, the stage IV sporulation protein B of endospore-forming bacteria such as Bacillus subtilis, is a serine proteinase, expressed in the spore (rather than mother cell) compartment, that participates in a proteolytic activation cascade for Sigma-K. It appears to be universal among endospore-forming bacteria and occurs nowhere else. [Cellular processes, Sporulation and germination]
Jiyao W et al.(2023). "The conserved domain database in 2023.",