# Mechanistic insights into SteAB regulation of cell wall hydrolase RipA in Mycobacterium tuberculosis

## TL;DR

This study reveals how the SteAB complex in Mycobacterium tuberculosis regulates the enzyme RipA to control cell wall breakdown during cell division.

## Contribution

The paper provides structural and functional insights into the SteAB complex's role in regulating RipA, challenging previous assumptions about MctB's function.

## Key findings

- The SteAB complex physically interacts with RipA to regulate cell wall hydrolase activity during cell division.
- Structural analysis shows that SteAB positions RipA to activate peptidoglycan hydrolysis.
- A knockout mutant analysis supports the model of SteAB as a regulator of cell separation in mycobacteria.

## Abstract

D,L-endopeptidase RipA is the major peptidoglycan (PG) hydrolase required for cell separation in Mycobacterium tuberculosis (Mtb), as RipA defects severely hinder cell division and increase antibiotic vulnerability. Despite extensive studies, the mechanisms governing Mtb RipA regulation remain controversial and poorly understood. Here, we report an integrative structural and functional analysis of the Mtb SteAB system, a regulatory transmembrane septal complex encoded by adjacent genes Rv1697 (steA) and Rv1698/mctB (steB) that are conserved across Mycobacteriales. The separate crystal structures of the cytoplasmic core of MtSteA and the periplasmic core of MtSteB in complex with the RipA coiled-coil domain (RipACC), along with biochemical evidence that these proteins form a stable transmembrane physical complex, define the function of the SteAB complex as a regulator of RipA-mediated cell separation in Mtb, arguing against a previously proposed role of MctB (Rv1698) as a putative mycobacterial outer membrane copper transporter. Our structural findings showed that, upon MtSteB binding, RipACC is oriented perpendicular to the membrane, bringing its endopeptidase catalytic domain in physical contact with the PG layer, while the homodimeric structural core of SteA revealed a conserved functional pocket similar to the phosphonucleotide-binding site of thiamine pyrophosphokinase. These data, coupled with the in vivo phenotypic analysis of a steAB knockout mutant of Cglu, support a model in which the SteAB heterotetramer orchestrates the productive positioning of RipA leading to PG hydrolysis activation. These findings shed new light on the regulation of mycobacterial cell wall remodeling, with implications for understanding Mtb pathogenesis and identifying novel antimicrobial targets.

Peptidoglycan (PG) is a major component of the bacterial cell wall. A flexible but strong PG mesh encloses the cell, conferring mechanical resistance and preventing cell lysis. This PG mesh is continually remodeled during the bacterial life by the coordinated action of tightly regulated PG-hydrolases and synthetases. Here, we report the structural characterization of the M. tuberculosis SteAB system, which regulates the action of the major enzyme responsible for disassembling the PG mesh to allow daughter cell separation at the end of cell division. The proposed model for the septal control of PG hydrolysis illustrates how the transmembrane SteAB complex can promote enzyme activation and provides structural information that may help target the activation mechanism for antibiotic development.