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Motab-Like Machinery Drives the Movement of Mreb Filaments During Bacterial Gliding Motility

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Motab-Like Machinery Drives the Movement of Mreb Filaments During Bacterial Gliding Motility MotAB-like machinery drives the movement of MreB filaments during bacterial gliding motility Guo Fua, Jigar N. Bandariab, Anne Valérie Le Gallc, Xue Fand, Ahmet Yildizb,e, Tâm Mignotc, David R. Zusmane,1, and Beiyan Nana,1 aDepartment of Biology, Texas A&M University, College Station, TX 77843; bDepartment of Physics, University of California, Berkeley, CA 94720; cLaboratoire de Chimie Bactérienne, UMR7283, Institut de Microbiologie de la Méditerranée, CNRS-Aix Marseille University, 13009 Marseille, France; dDepartment of Statistics, Texas A&M University, College Station, TX 77843; and eDepartment of Molecular and Cell Biology, University of California, Berkeley, CA 94720 Edited by Christine Jacobs-Wagner, Yale University, West Haven, CT, and approved January 22, 2018 (received for review September 19, 2017) MreB is a bacterial actin that is important for cell shape and cell Visualized by regular fluorescent microscopy, proteins in the wall biosynthesis in many bacterial species. MreB also plays crucial gliding machinery appeared either as blurry patches that moved in roles in Myxococcus xanthus gliding motility, but the underlying the cell envelopes or bright aggregates that remained relatively sta- mechanism remains unknown. Here we tracked the dynamics of tionary at the focal adhesion sites (6, 15, 16, 18). Thus, single-particle single MreB particles in M. xanthus using single-particle tracking tracking photoactivated localization microscopy (sptPALM) photoactivated localization microscopy. We found that a subpop- was used to clarify the dynamics of motility-related proteins at ulation of MreB particles moves rapidly along helical trajectories, subdiffraction resolutions. sptPALM analyses of AglR, a MotA ho- similar to the movements of the MotAB-like gliding motors. The molog, revealed that the motor subunits moved along helical tra- rapidMreBmotionwasstalledinthemutantsthatcarriedtruncated jectories (5). Consistent with this observation, GltD (AgmU) and gliding motors. Remarkably, M. xanthus MreB moves one to two AglR decorate a structure that appears helical in fixed cells, but the exact composition of this structure has remained elusive (5, 6). orders of magnitude faster than its homologs that move along with Recent studies suggest that the directionality of the motility complex the cell wall synthesis machinery in Bacillus subtilis and Escherichia depends on its interactions with three key cytoplasmic components: coli, and this rapid movement was not affected by the inhibitors of M. xanthus MreB, the Ras-like GTPase MglA, and a PilZ-like regulator, PlpA cell wall biosynthesis. Our results show that in ,MreB (19–21). Among these proteins, the direct interaction between MreB provides a scaffold for the gliding motors while the gliding machinery and MglA-GTP regulates the spatial assembly, disassembly, and di- MICROBIOLOGY drives the movement of MreB filaments, analogous to the interde- rectionality of the Agl-Glt complexes (19, 20). pendent movements of myosin motors and actin in eukaryotic cells. M. xanthus gliding motility requires functional MreB filaments. The MreB inhibitor A22 [S-(3,4-dichlorobenzyl) isothiourea] blocks bacterial cytoskeleton | flagella stator homolog | protein dynamics | thegliding,butfailstodosointhecellsthatexpressanA22-resistent super-resolution microscopy | peptidoglycan synthesis MreB variant (4–6). This result suggests that A22 inhibits gliding motility specifically through MreB. Nonetheless, the precise function(s) ctin, the most abundant protein in the human body, forms of MreB in gliding motility remain unclear. MreB could connect to Alinear filaments to provide cells with mechanical support the motility complex indirectly through MglA (19, 20); however, and participates in many cellular processes (1). In eukaryotic this connection has not been directly observed in motile cells. In cells, myosin motors generate force along actin, enabling move- this study, we constructed a functional photoactivatable MreB fu- ments required for muscle contraction, motility, cell division, and sion and investigated its dynamics at single-particle resolution. We show that MreB provides a scaffold for the gliding motors while intracellular transport (1). Actin-like proteins have also been discovered in bacteria. The bacterial actin MreB participates in a wide range of functions, including cell shape determination, cell Significance wall biosynthesis, and motility (2–6). In rod-shaped bacteria, MreB guides the insertion of new cell wall material around the long axis MreB is a bacterial actin that supports cell wall synthesis and rod- of cells (7). In Bacillus subtilis and Escherichia coli, MreB filaments like morphology in many bacteria. In the bacterium Myxococcus localize into punctate patterns and appear to form short filaments. xanthus,MreBisalsorequiredforglidingmotility.Inthiswork, Although the conformation of MreB filaments is still open to we studied the movement of MreB particles during M. xanthus discussion, it is widely agreed that MreB is highly dynamic in most gliding motility at high spatial and temporal resolution. We found bacterial species (8–13). For example, MreB filaments rotate that MreB provides a scaffold for the gliding motors while the slowly (∼1rpm,or10–90 nm/s) around the long cell axis of gliding machinery drives the movement of MreB filaments. The B. subtilis and E. coli. Such movement is driven by the peptido- interplay between MreB and the gliding machineries is analogous glycan (PG) synthesis machinery and blocked by the addition of to the interdependent movements of myosin motors and actin in antibiotics that inhibit PG synthesis (8–10). eukaryotic cells. Since their discovery, actin-like homologs have MreB also plays a unique role in the gliding motility of Myxococcus been implicated in many spatially organized cellular processes in xanthus, a rod-shaped, nonflagellated, Gram-negative soil bacterium. bacteria. This study expands our knowledge of MreB function in M. xanthus is able to glide along solid substrates without the aid of bacterial gliding motility. type IV pili. Its gliding motility is powered by the action of the Agl- Glt complex, which contains up to 17 proteins, including cytosolic, Author contributions: B.N. designed research; G.F., J.N.B., A.V.L.G., and B.N. performed inner membrane, periplasmic, and outer membrane components research; A.V.L.G. and T.M. contributed new reagents/analytic tools; G.F., X.F., A.Y., and (14, 15). Following its initial assembly at the leading cell pole, the B.N. analyzed data; and G.F., A.Y., T.M., D.R.Z., and B.N. wrote the paper. Agl-Glt complex further assembles into a force-generating unit The authors declare no conflict of interest. through interaction with AglRQS, a proton channel complex ho- This article is a PNAS Direct Submission. mologous to the E. coli flagella stator complex MotAB (5, 15–17) Published under the PNAS license. and GltG/I/J, which form a putative inner membrane platform. 1To whom correspondence may be addressed. Email: [email protected] or bnan@ Assembled force-generating units move directionally toward the tamu.edu. lagging cell pole following rotational trajectories and propel a ro- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. tational movement of the cell when they engage bacterial focal 1073/pnas.1716441115/-/DCSupplemental. adhesions with the underlying surface (5, 17). www.pnas.org/cgi/doi/10.1073/pnas.1716441115 PNAS Latest Articles | 1of6 Downloaded by guest on September 28, 2021 the gliding machinery drives the movement of MreB filaments. absence of copper, indicating that this MreB-PAmCherry fu- The interdependence between MreB and the gliding machineries sion was functional (Fig. 1 D and E and SI Appendix,TableS1). reveal a direct function of MreB in myxobacterial gliding motility. This MreB-PAmCherry fusion appeared as a single band cor- responding to its molecular weight (∼50 kDa), indicating that Results the fusion protein was stably expressed (Fig. 1F). Isolation of an M. xanthus Strain Expressing a Functional MreB- PAmCherry. We constructed a M. xanthus strain that expresses MreB Filaments Exhibit Rapid Dynamic Movements in M. xanthus Cells. MreB fused to photoactivatable mCherry (PAmCherry) to image We used fluorescence microscopy to track MreB-PAmCherry in the entire cellular MreB pool as well as single MreB particles. moving cells. When exposed to 405-nm excitation (0.2 kW/cm2)for Because both the N and C termini of MreB participate in the po- 2 s, the majority of MreB-PAmCherry was photoactivated, appearing lymerization of filaments (22, 23) and are sensitive to structural as small patches along the cell body, similar to the localization de- perturbation (SI Appendix, Fig. S1) (24), we inserted PAmCherry scribed for other bacteria (Fig. 2A and SI Appendix,Fig.S2). We at various positions in the internal loops of MreB to minimize recorded the dynamics of the fluorescent patches by time-lapse mi- interference with function (SI Appendix, Table S1). To screen the croscopy at 10 Hz using highly inclined and laminated optical sheet MreB-PAmCherry constructs for functionality, we constructed a illumination (26). The MreB filaments in M. xanthus showed rapid conditional mreB depletion strain, as mreB is essential for M. irregular motion both across the cell width and along the long cell xanthus viability. In this depletion strain, mreB was expressed ectopi- axis, in contrast to the slow circumferential movements observed in – cally from a copper-inducible
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