putative domain B insert of mltA_type lytic transglycosylases
Escherichia coli MltA is a membrane-bound lytic transglycosylase comprised of two domains separated by a large groove, where the peptidoglycan strand binds. Domain A is made up of an N-terminal and a C-terminal portion, which correspond to the 3D domain, named for 3 conserved aspartate residues. Domain B is inserted within the linear sequence of domain A. MltA is distinct from other bacterial lytic transglycosylases (LTs), which are similar to each other. Escherichia coli peptidoglycan lytic transglycosylase (LT) initiates cell wall recycling in response to damage, during bacterial fission, and cleaves peptidoglycan (PG) to create functional spaces in its wall. PG chains (also known as murein), the major components of the bacterial cell wall, are comprised of alternating beta-1-4-linked N-acetylmuramic acid (MurNAc) and N-acetyl-D-glucosamine (GlcNAc), and lytic transglycosylases cleave this beta-1-4 bond. Typically, peptidoglycan lytic transglycosylases (LT) are exolytic, releasing Metabolite 1 (GlcNAc-anhMurNAc-L-Ala-D-Glu-m-Dap-D-Ala-D-Ala) from the ends of the PG strands. In contrast, MltE is endolytic , cleaving in the middle of PG strands, with further processing to Metabolite 1 accomplished by other LTs. In E. coli, there are six membrane-bound LTs: MltA-MltF and soluble Slt70. Slt35 is a soluble fragment cleaved from MltB. Bacterial LTs are classified in 4 families: Family 1 includes slt70 MltC-MltF, Family 2 includes MltA, Family 3 includes MltB, and Family 4 of bacteriophage origin. While most of the LT family members are similar in structure and sequence with a lysozyme-like fold, Family 2 (including mltA) is distinct.
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