MreB Drives De Novo Rod Morphogenesis in Caulobacter crescentus via Remodeling of the Cell Wall

MreB, the bacterial actin-like cytoskeleton, is required for the rod morphology of many bacterial species. Disruption of MreB function results in loss of rod morphology and cell rounding. Here, we show that the widely used MreB inhibitor A22 causes MreB-independent growth inhibition that varies with the drug concentration, culture medium conditions, and bacterial species tested. MP265, an A22 structural analog, is less toxic than A22 for growth yet equally efficient for disrupting the MreB cytoskeleton. The action of A22 and MP265 is enhanced by basic pH of the culture medium. Using this knowledge and the rapid reversibility of drug action, we examined the restoration of rod shape in lemon-shaped Caulobacter crescentus cells pretreated with MP265 or A22 under nontoxic conditions. We found that reversible restoration of MreB function after drug removal causes extensive morphological changes including a remarkable cell thinning accompanied with elongation, cell branching, and shedding of outer membrane vesicles. We also thoroughly characterized the composition of C. crescentus peptidoglycan by high-performance liquid chromatography and mass spectrometry and showed that MreB disruption and recovery of rod shape following restoration of MreB function are accompanied by considerable changes in composition. Our results provide insight into MreB function in peptidoglycan remodeling and rod shape morphogenesis and suggest that MreB promotes the transglycosylase activity of penicillin-binding proteins.Most bacteria have characteristic cell morphologies maintained during growth (67). The peptidoglycan (PG) component of the cell wall represents in most cases the physical support of various bacterial shapes. PG is a mesh-like polymeric macromolecule which opposes the osmotic pressure of the bacterial cytoplasm and prevents lysis in hypotonic growth environments (29). Isolated PG cell walls (sacculi) retain the shapes of the cells from which they originate while PG disruption causes the formation of osmotically labile spheroplasts, underscoring PG''s essential role in cell shape determination and cellular integrity maintenance. PG is composed of long glycan chains that are oriented roughly along the short axis of rod-shaped Gram-negative bacteria and that are connected by short peptide cross-links (21, 60). Bacterial growth and division necessitate the expansion and division of the PG cell wall, which requires the insertion of new PG material in the preexisting, covalently linked mesh (29). New PG synthesis requires two enzymatic reactions performed by penicillin-binding proteins (PBPs). Glycan chain synthesis is achieved by transglycosylation activity while cross-linkage of glycan chains to the existing mesh is achieved by transpeptidation activity (47). Class A PBPs, called bifunctional or bimodular PBPs (e.g., PBP1a and 1b of Escherichia coli), possess both transpeptidase and transglycosylase domains while class B PBPs, such as PBP2 and PBP3 of E. coli, can perform only transpeptidase reactions (47). Controlled degradation of the PG by cell wall hydrolases is necessary for incorporation of new PG material during growth. Tight coordination between PG synthesis and degradation is required to maintain the integrity of the mesh at all times (29).The bacterial cytoskeleton also plays a central role in cell shape determination and maintenance (7). MreB is a bacterial actin homolog that forms dynamic helical structures underneath the cytoplasmic membrane in most rod-shaped bacteria (8, 34, 37, 56). In some species, the spatial distribution of MreB varies during the cell cycle, changing from a helical/patchy localization pattern throughout the cell to a ring-like distribution near midcell (20, 22, 50, 58). MreB is required for rod shape maintenance as deletion of the MreB-encoding gene or depletion of MreB causes loss of rod shape and cell rounding (20, 22, 34, 63). Other proteins, including MreC, MreD, RodA, PBP2, and RodZ, function along with MreB to maintain rod shape as loss of their function also results in cell rounding (2, 5, 33, 48, 62). Among these rod-morphogenic proteins, only PBP2 has a known enzymatic function, being involved in PG synthesis as an elongation-specific transpeptidase; the others are membrane-spanning or integral membrane proteins (2, 5, 15, 48). The overall involvement of these morphogenetic proteins in rod shape maintenance has led to a model in which they are part of the elongase complex, a PG synthesizing machine that elongates the PG side wall (2, 5, 15, 48, 59). The elongase complex would include PG lytic enzymes and at least one bifunctional PBP required for glycan strand synthesis (15, 59). In Bacillus subtilis, MreB homologs were found to associate with the bifunctional PBP1 (36) and to regulate the localization of the PG hydrolase LytE (9). However, it is still unclear how MreB functions in the context of the proposed elongase complex to determine and maintain rod shape.It has been previously shown that repletion of MreB in lemon-shaped, MreB-depleted Caulobacter crescentus cells leads to the formation of cell filaments that present branches and ectopic stalks (64). To examine how MreB can drive de novo rod shape morphogenesis, we followed a similar strategy except that we used drug treatment to interfere with MreB function. The small molecule 3,4-dichlorobenzyl carbamimidothioate, also known as A22, has been shown to rapidly disrupt MreB localization in vivo and to induce growth-dependent rounding in several Gram-negative bacteria (23, 32, 41, 45, 52). Furthermore, genetic and biochemical experiments have shown that MreB is the direct molecular target of A22 and that A22 binds to MreB''s ATP-binding pocket, inducing a state with low affinity for polymerization (3, 23). As removal of A22 is followed within minutes by recovery of the normal MreB localization pattern (23), this drug represents a convenient tool for rapid and reversible inhibition of MreB function. However, A22 was found to inhibit the growth of an mreB deletion mutant of E. coli, suggesting that it can have MreB-independent toxic effects (35). In this study, we show that the toxicity of A22 varies with the drug concentration, culture medium conditions, and Gram-negative species tested. We identify a similarly potent but less toxic structural analog, MP265 (4-chlorobenzyl carbamimidothioate), as well as nontoxic concentrations and conditions for both A22 and MP265 that induce loss of rod cell morphology in C. crescentus. We also show that recovery of rod shape after drug removal is accompanied by intensive remodeling of PG morphology and composition.