Super-resolution Imaging of the Bacterial Division Machinery

Summary

We describe a super-resolution imaging method to probe the structural organization of the bacterial FtsZ-ring, an essential apparatus for cell division. This method is based on quantitative analyses of photoactivated localization microscopy (PALM) images and can be applied to other bacterial cytoskeletal proteins.

Abstract

Bacterial cell division requires the coordinated assembly of more than ten essential proteins at midcell^1,2. Central to this process is the formation of a ring-like suprastructure (Z-ring) by the FtsZ protein at the division plan^3,4. The Z-ring consists of multiple single-stranded FtsZ protofilaments, and understanding the arrangement of the protofilaments inside the Z-ring will provide insight into the mechanism of Z-ring assembly and its function as a force generator^5,6. This information has remained elusive due to current limitations in conventional fluorescence microscopy and electron microscopy. Conventional fluorescence microscopy is unable to provide a high-resolution image of the Z-ring due to the diffraction limit of light (~200 nm). Electron cryotomographic imaging has detected scattered FtsZ protofilaments in small C. crescentus cells⁷, but is difficult to apply to larger cells such as E. coli or B. subtilis. Here we describe the application of a super-resolution fluorescence microscopy method, Photoactivated Localization Microscopy (PALM), to quantitatively characterize the structural organization of the E. coli Z-ring⁸.

PALM imaging offers both high spatial resolution (~35 nm) and specific labeling to enable unambiguous identification of target proteins. We labeled FtsZ with the photoactivatable fluorescent protein mEos2, which switches from green fluorescence (excitation = 488 nm) to red fluorescence (excitation = 561 nm) upon activation at 405 nm⁹. During a PALM experiment, single FtsZ-mEos2 molecules are stochastically activated and the corresponding centroid positions of the single molecules are determined with <20 nm precision. A super-resolution image of the Z-ring is then reconstructed by superimposing the centroid positions of all detected FtsZ-mEos2 molecules.

Using this method, we found that the Z-ring has a fixed width of ~100 nm and is composed of a loose bundle of FtsZ protofilaments that overlap with each other in three dimensions. These data provide a springboard for further investigations of the cell cycle dependent changes of the Z-ring¹⁰ and can be applied to other proteins of interest.

Protocol

1. Sample Preparation

1. Inoculate LB media with a single colony of strain JB281 [BW25113 / pJB042 (P_Lac:FtsZ-mEos2)]. Grow overnight in a shaker at 37 °C.
2. Dilute the culture 1:1,000 into M9⁺ minimal media [M9 Salts, 0.4% Glucose, 2 mM MgSO₄, 0.1 mM CaCl₂, MEM Amino Acids and Vitamins] and grow to mid-log phase (OD₆₀₀= 0.2-0.3) in the presence of chloramphenicol (150 μg/ml) at room temperature (RT).
3. Induce culture with 20 μM IPTG for 2 hr (see Note #1).
4. Pellet culture in microcentrifuge (8,000 rpm, 1 min) and resuspend in an equal volume of M9⁺; repeat. A....

Results

Illustrated in Figure 3Aiv is a two-dimensional, super-resolution rendering of the Z-ring generated from the PALM imaging method described above. Below, we summarize qualitative and quantitative information that can be obtained from them.

Qualitatively, we observed that the Z-ring is an irregular structure that adopts multiple configurations (single band or helical arc) that are not distinguishable in conventional fluorescence images (compare Figure 3A-Dii

Discussion

PALM images contain information about molecule counts and positions within a cell, allowing detailed analysis of the distribution and arrangement of target protein molecules that is difficult to achieve by other means. Below we outline precautions that should be taken to extract accurate quantitative information while maintaining the biological relevance of PALM images. We also explore the information that can be best obtained using live vs. fixed cells. Finally, we suggest avenues for obtaining additional super-.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
50 x MEM Amino Acids	Sigma	M5550
100 x MEM Vitamins	Sigma	M6895
IPTG	Mediatech	46-102-RF
16% Paraformaldehyde	Electron Micrsocopy Sciences	15710-S
SeaPlaque GTG Agarose	Lonzo	50111
50 nm Gold Beads	Microspheres-Nanospheres	790113-010
FCS2 Imaging Chamber	Bioptechs
Stage Adaptor	ASI	I-3017
Inverted Microscope	Olympus	IX71
1.45 NA, 60x Objective	Olympus
IXON EMCCD Camera	Andor Technology	DU897E
488-nm Sapphire Laser	Coherent
561-nm Sapphire Laser	Coherent
405-nm CUBE Laser	Coherent