A Dynamically Assembled Cell Wall Synthesis Machinery Buffers Cell Growth
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A dynamically assembled cell wall synthesis machinery buffers cell growth Timothy K. Leea, Carolina Tropinia,b, Jen Hsina, Samantha M. Desmaraisa, Tristan S. Ursella, Enhao Gonga, Zemer Gitaic, Russell D. Mondsa,1,2, and Kerwyn Casey Huanga,b,d,1 aDepartment of Bioengineering and bBiophysics Program, Stanford University, Stanford, CA 94305; cDepartment of Molecular Biology, Princeton University, Princeton, NJ 08544; and dDepartment of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305 Edited by Carol A. Gross, University of California, San Francisco, CA, and approved January 15, 2014 (received for review July 30, 2013) Assembly of protein complexes is a key mechanism for achieving support for the hypothesis that coordination between MreB and spatial and temporal coordination in processes involving many the wall synthesis machinery is achieved through colocalization enzymes. Growth of rod-shaped bacteria is a well-studied example into large, moving, multienzyme complexes (4, 5). However, bio- requiring such coordination; expansion of the cell wall is thought chemical isolation of such a complex has remained elusive (6), to involve coordination of the activity of synthetic enzymes raising the possibility that this picture is incomplete, particularly with the cytoskeleton via a stable complex. Here, we use single- in E. coli, in which dynamic imaging of the PBPs has not yet molecule tracking to demonstrate that the bacterial actin homolog been achieved. MreB and the essential cell wall enzyme PBP2 move on timescales PBP2 is thought to be the major enzyme responsible for co- orders of magnitude apart, with drastically different characteristic valently cross-linking new glycan strands into the cell wall during motions. Our observations suggest that PBP2 interacts with growth (7). PBP2 is essential; cells enlarge and eventually lyse the rest of the synthesis machinery through a dynamic cycle of upon depletion of PBP2 (8). In E. coli, PBP2 was previously transient association. Consistent with this model, growth is robust observed to localize as dispersed puncta and at septa during to large fluctuations in PBP2 abundance. In contrast to stable division (9). PBP2 concentration affects cell size (10), and complex formation, dynamic association of PBP2 is less dependent overexpression is lethal (11). Although MreB binds to the cyto- on the function of other components of the synthesis machinery, plasmic face of the inner membrane, PBP2 is a transmembrane and buffers spatially distributed growth against fluctuations in protein with a cytoplasmic N-terminus and a periplasmic domain MICROBIOLOGY pathway component concentrations and the presence of defective required for transpeptidase activity. Given the requirement of components. Dynamic association could generally represent an a functional PBP2 for MreB motion and peptidoglycan synthesis, efficient strategy for spatiotemporal coordination of protein activi- we sought to determine whether it could be a component of a ties, especially when excess concentrations of system components cell wall-synthesizing, multienzyme complex moving with MreB. are inhibitory to the overall process or deleterious to the cell. In this study, we applied single-particle tracking photoactivated localization microscopy (sptPALM) (12), a technique based Pencillin binding proteins | bacterial cell wall | multienzyme complexes | on the limited photoactivation of fluorescent proteins, to quan- superresolution microscopy tify the dynamics of MreB and PBP2 in E. coli. In contrast to MreB, we find that PBP2 exhibits rapid, diffusive motions that or the wide variety of organisms that have large internal os- do not depend on PBP2 catalytic activity. These data suggest Fmotic pressures, the cell wall is essential for maintaining cellular integrity and morphology (1). As a consequence, the Significance bacterial cell wall is an important antibiotic target. Much re- search has been performed to elucidate the chemical composi- For complex biological processes, the formation of protein tion of the cell wall and identify the enzymes required for its complexes is a strategy for coordinating the activities of many synthesis, yet we still know little about the dynamics of cell wall enzymes in space and time. It has been hypothesized that assembly in vivo. The cell wall is composed of peptidoglycan, growth of the bacterial cell wall involves stable synthetic a network of long glycan strands that are cross-linked by short complexes, but neither the existence of such complexes nor the peptides. During growth, existing bonds in the peptidoglycan consequences of such a mechanism for growth efficiency have network are broken, and new glycans are polymerized, inserted, been demonstrated. Here, we use single-molecule tracking to and cross-linked. This process depends on a host of enzymes, demonstrate that the association between an essential cell most notably the penicillin-binding proteins (PBPs) (2). It is wall synthesis enzyme and the cytoskeleton is highly dynamic, presumed that the spatial and temporal coordination between which allows the cell to buffer growth rate against large fluc- these proteins is responsible for the maintenance of cell shape tuations in enzyme abundance. This indicates that dynamic during growth and division, although the strategies used to ro- association can be an efficient strategy for coordination of bustly achieve such a task that spans both molecular and cellular multiple enzymes, especially those for which excess abundance scales have not yet been elucidated. can be harmful to cells. Recent live-cell imaging studies in the rod-shaped bacteria Escherichia coli (3) and Bacillus subtilis (4, 5) showed that clus- Author contributions: T.K.L., C.T., J.H., S.M.D., Z.G., R.D.M., and K.C.H. designed research; ters of MreB, a bacterial actin homolog that is essential for rod- T.K.L., C.T., S.M.D., and E.G. performed research; T.K.L., C.T., J.H., S.M.D., T.S.U., R.D.M., and K.C.H. contributed new reagents/analytic tools; T.K.L., C.T., J.H., S.M.D., and K.C.H. like shape, move in linear tracks oriented in the circumferential analyzed data; and T.K.L., C.T., J.H., S.M.D., T.S.U., Z.G., R.D.M., and K.C.H. wrote direction. These dynamics are coupled to cell wall synthesis; the paper. exposure to cell wall-targeting antibiotics or depletion of cell wall The authors declare no conflict of interest. – E. coli precursors halts MreB motion (3 5). In , MreB motion was This article is a PNAS Direct Submission. β stopped by exposure to high concentrations of the -lactam an- 1To whom correspondence may be addressed. E-mail: [email protected] or rmonds@ tibiotic mecillinam, which inhibits the transpeptidase PBP2 (3), syntheticgenomics.com. suggesting a model in which PBP2 activity is necessary for MreB 2Present address: Synthetic Genomics Inc., La Jolla, CA 92037. B. subtilis motion, possibly by directing new cell wall insertion. In , This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. tracking of MreB-associated proteins (MreC/D) and PBPs provided 1073/pnas.1313826111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1313826111 PNAS Early Edition | 1of6 Downloaded by guest on September 29, 2021 a model in which PBP2 transiently associates with sites of cell proteins (15), particularly those in low abundance, these results wall synthesis, and hence can act in a distributed manner and indicate that the circumferential motion of single molecules is a need not be rate limiting for growth as was previously hypothe- requisite signature for colocalization with MreB. sized (3). In support of this model, growth was unaffected for more than two doublings during depletion of PBP2. Finally, we Rapid, Diffusive Motion of PBP2 Molecules. To determine the dy- show that both growth rate and MreB speed decrease during namics of PBP2 in E. coli, we constructed an N-terminal fusion mecillinam treatment in a dose-dependent manner, indicating of PAmCherry to PBP2 expressed from the endogenous pro- that a catalytically active PBP2 molecule is required during the moter and verified that this fusion rescued cells from depletion D–G incorporation of a glycan strand. of the native, unlabeled PBP2 (Fig. S1 ). As the sole source of PBP2, either expressed from a low-copy plasmid or integrated Results at the native chromosomal locus, PAmCherry-PBP2 complemented Single-Molecule Tracking of MreB Circumferential Motion in E. coli. We wild-type growth rate with slightly higher average cell width F G tagged E. coli MreB with PAmCherry (13) as an internal sandwich (Fig. S1 and ). Bocillin gels and Western blots demonstrated Materials fusion (14) and expressed this fusion (MreBsw-PAmCherry) as that the fusion remained stable and intact inside the cell ( and Methods A the sole copy at the native chromosomal locus. E. coli cells ; Figs. S3 and S4 ). expressing MreBsw-PAmCherry fusions were viable and remained We observed striking qualitative and quantitative differences rod-shaped, although cells were slightly wider (Fig. S1 A–C), as between the motions of MreB and PBP2. PBP2 molecules moved previously observed for mCherry sandwich fusions (14). We much faster and more erratically than MreB. Reconstructing the imaged single cells using total internal reflectance fluorescence trajectory of a PBP2 molecule required imaging on a timescale 100 times faster (∼10 ms) than that required to observe linear (TIRF) microscopy through several hundred iterations of a cycle Materials and Methods in which a single molecule was activated, followed by time-lapse MreB trajectories ( ; Fig. S5). Moreover, imaging of the molecule’s dynamics every 2.5 s until it photo- the mean squared displacement (MSD) of PBP2 motion was linear (Fig. 1D), indicating diffusive behavior with an apparent bleached (Fig. 1A, Fig. S2, and Materials and Methods). Our diffusion constant of 0.06 ± 0.006 μm2/s. The difference in the sptPALM measurements revealed directed, circumferential mobilities of MreB and PBP2 excluded the possibility that MreB motion of single MreB molecules (Fig.