Structure and reconstitution of a hydrolase complex explain how nascent peptidoglycan is released from the membrane during cell wall assembly

Bacteria are surrounded by a peptidoglycan cell wall that is essential for their survival. During cell wall assembly, a lipid-linked disaccharide-peptide precursor called Lipid II is polymerized and crosslinked to produce mature peptidoglycan. As Lipid II is polymerized, nascent polymers remain membrane-anchored at one end and the other end becomes crosslinked to the matrix. A longstanding question is how bacteria release newly synthesized peptidoglycan strands from the membrane to complete the synthesis of mature peptidoglycan. Here we show that a Staphylococcus aureus cell wall hydrolase and a membrane protein containing eight transmembrane helices form a complex that acts as a peptidoglycan release factor. This complex cleaves nascent peptidoglycan internally to produce free oligomers as well as lipid-linked oligomers that can undergo further elongation. The membrane protein partner controls the lengths of these products. We solved a 2.6 Å resolution structure of the complex that shows that the membrane protein scaffolds the hydrolase to orient its active site for cleavage of the glycan strand. We propose that this complex serves to detach newly-synthesized peptidoglycan polymer from the cell membrane to complete integration into the cell wall matrix.