A cytoskeleton-like role for the bacterial cell wall during engulfment of the Bacillus subtilis forespore

Abstract

A hallmark of bacterial endospore formation is engulfment, during which the membrane of one cell (the mother cell) migrates around the future spore, enclosing it in the mother cell cytoplasm. Bacteria lack proteins required for eukaryotic phagocytosis, and previously proteins required for membrane migration remained unidentified. Here we provide cell biological and genetic evidence that three membrane proteins synthesized in the mother cell are required for membrane migration as well as for earlier steps in engulfment. Biochemical studies demonstrate that one of these proteins, SpoIID, is a cell wall hydrolase, suggesting that membrane migration in bacteria can be driven by membrane-anchored cell wall hydrolases. We propose that the bacterial cell wall plays a role analogous to that of the actin and tubulin network of eukaryotic cells, providing a scaffold along which proteins can move.[Keywords: Bacillus subtilis; sporulation; membrane movement; peptidoglycan hydrolysis; protein localization] Supplemental material is available at http://www.genesdev.org.

Cell-wall remodeling drives engulfment during Bacillus subtilis sporulation

“…It is unclear how the PBPs recognize the LE, as localization of forespore produced GFP-PonA and GFP-PbpA did not depend on candidate proteins SpoIIB, SpoIID, SpoIIM, SpoIIP, SpoIIQ, SpoIIIAH, SpoIVFAB, or GerM (Aung et al, 2007; Abanes-De Mello et al, 2002; Chastanet and Losick, 2007; Blaylock et al, 2004; Rodrigues et al, 2016) (Figure 2—figure supplement 2). However, these results indicate that the forespore plays a critical role in PG synthesis, and point to an engulfment mechanism that does not depend on pulling the engulfing membrane by mother cell-directed peptidoglycan synthesis.…”

“…This complex is static and is proposed to act as a Brownian ratchet to prevent backwards movement of the engulfing membrane, contributing to the robustness of engulfment in intact cells (Sun et al, 2000; Broder and Pogliano, 2006). Second, the SpoIID, SpoIIM and SpoIIP complex (DMP) localizes at the leading edge (LE) of the mother cell engulfing membrane and is essential and rate limiting for membrane migration (Abanes-De Mello et al, 2002; Gutierrez et al, 2010). The complex contains two enzymes that degrade PG in a processive manner: SpoIIP removes stem peptides, and SpoIID degrades the resulting denuded glycan strands (Abanes-De Mello et al, 2002; Chastanet and Losick, 2007; Morlot et al, 2010; Gutierrez et al, 2010).…”

“…Second, the SpoIID, SpoIIM and SpoIIP complex (DMP) localizes at the leading edge (LE) of the mother cell engulfing membrane and is essential and rate limiting for membrane migration (Abanes-De Mello et al, 2002; Gutierrez et al, 2010). The complex contains two enzymes that degrade PG in a processive manner: SpoIIP removes stem peptides, and SpoIID degrades the resulting denuded glycan strands (Abanes-De Mello et al, 2002; Chastanet and Losick, 2007; Morlot et al, 2010; Gutierrez et al, 2010). Mutants with reduced SpoIID or SpoIIP activity or protein levels engulf asymmetrically, with the engulfing membrane migrating faster on one side of the forespore (Abanes-De Mello et al, 2002; Gutierrez et al, 2010).…”

“…The complex contains two enzymes that degrade PG in a processive manner: SpoIIP removes stem peptides, and SpoIID degrades the resulting denuded glycan strands (Abanes-De Mello et al, 2002; Chastanet and Losick, 2007; Morlot et al, 2010; Gutierrez et al, 2010). Mutants with reduced SpoIID or SpoIIP activity or protein levels engulf asymmetrically, with the engulfing membrane migrating faster on one side of the forespore (Abanes-De Mello et al, 2002; Gutierrez et al, 2010). Third, blocking PG precursor synthesis with antibiotics impairs membrane migration in mutants lacking the Q-AH zipper, suggesting that PG synthesis at the LE of the engulfing membrane contributes to engulfment (Meyer et al, 2010; Tocheva et al, 2013).…”

“…The outer cell wall of Gram-positive bacteria also contains a significant amount of teichoic acids, important for cell morphology, phosphates, and antibiotic resistance (Grant, 1979; Brown et al, 2013) but largely absent in spores (Chin et al, 1968; Johnstone et al, 1982). Engulfment entails extensive cell wall remodeling, with peptidoglycan precursors, newly synthesized PG and the sporulation specific PG degradation machinery localizing at the LE of the engulfing membrane (Meyer et al, 2010; Tocheva et al, 2013; Abanes-De Mello et al, 2002). However, since engulfment occurs at high turgor pressure within the cramped confines of the thick outer cell wall, we expect that membrane movement is severely reduced by steric hindrance (Lizunov and Zimmerberg, 2006).…”