PrgK has three extracellular domains. We here show that only one of the three domains is active, and in contrast to other known homologs in T4SSs it is actually a muramidase (MUR), instead of the proposed soluble lytic transglycosylase (SLT).

The other two domains, LytM and CHAP, are instead involved in regulating the MUR domain. This results in a self-regulation of PrgK, which is likely very important to prevent cell toxicity. Interestingly, this self-regulation is lost when acting on cell walls from Gram-negatives.