SAR endolysin

The SAR Paradigm

Signal Anchor Release (SAR) endolysins muralytic enzymes encoded by dsDNA phages that infect gram negative hosts. They are signaled to the host cytoplasmic membrane and are anchored there in an inactive conformation. They are maintained in the energized membrane and are released by the depolarization action of the holin. After release, the SAR domain folds into the soluble form of the enzyme, activating it.

The first SAR endolysin to be experimentally characterized was P1 from Enterobacteria phage P1. Cells expressing plasmid borne Lyz lyse independently of a holin suggesting that the protein is secreted to the periplasm. Indeed, Lyz mediated lysis is prevented by the addition of azide, a SecA inhibitor. A sub-cellular fractionation of cells expressing Lyz showed that the protein was partitioned both into the membrane and soluble fractions of the cell. The apparent molecular masses, however, of soluble and membrane bound Lyz were the same, indicating that the secreted protein is not the result of signal peptide cleavage. Taken together, the data indicate that the 20-30 amino acid N-terminal domain is responsible for: a) secretion of the protein through the Sec pathway, b) tethering the protein to the cytoplasmic membrane, and c) releasing the protein into the periplasm.

References

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