Unlike T4 E phage lysozyme, the endolysin R21 from Enterobacteria phage P21 has an N-terminal SAR (signal-arrest-release) domain that anchors the endolysin to the membrane in an inactive form, which act to prevent premature lysis of the infected bacterium. The dsDNA phages of eubacteria use endolysins or muralytic enzymes in conjunction with hollin, a small membrane protein, to degrade the peptidoglycan found in bacterial cell walls. Similarly, bacteria produce autolysins to facilitate the biosynthesis of its cell wall heteropolymer peptidoglycan and cell division. Endolysins and autolysins are found in viruses and bacteria, respectively. Both endolysin and autolysin enzymes cleave the glycosidic beta 1,4-bonds between the N-acetylmuramic acid and the N-acetylglucosamine of the peptidoglycan.
Jiyao W et al.(2023). "The conserved domain database in 2023.",