Geometric Cue for Protein Localization in a Bacterium

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Entry by Angelo Mao, AP 225, Fall 2010

Title: Geometric Cue for Protein Localization in a Bacterium

Authors: Kumaran S. Ramamurthi, Sigolene Lecuyer, Howard A. Stone, Richard Losick

Journal: Science

Volume: Vol 323

Pages: 1354-1356


Summary

The researchers elucidated the mechanisms behind the phenomenon of the localization of a bacteria protein SpoVM (VM) to convexly curved membranes. Through a series of experiments, the researchers concluded that it was the convexity of the cell membrane that attracted VM. They also proposed several possible explanations for why geometry was a necessary and sufficient cue for localization.

Soft Matter Keywords: bacteria, energetics, curvature

Curvature is implicated

Mao5 1.jpg

Convex membranes occur in Bacillus subtilis during spore formation, as shown in A. The spore is involuted into the mother cells, and VM adheres to the outer membrane of the spore. In the middle column of images, researchers compare the localization of VM-GFP (VM tagged with green fluorescent protein) with a mutant variety that localized promiscuously. The rightmost column of images show that cues from the plasma membrane are not responsible for VM's localization. Even when the membrane of the spore is sealed off (as shown by a dye not being able to access the membranes, in images H and K), wild type VM still localized to the convex membranes.

Positive curvature is necessary and sufficient

Mao5 2.jpg

The researchers demonstrated that positive curvature was necessary and sufficient for VM localization. Mutant Bacillus subtilis whose spore formation halted in the polar septum stage lacked positive curvature, and wildtype VM localization was promiscuous (left set of images). In yeast cells mutated to express VM (right set of images), VM localized to convex-surfaced vacuoles inside the yeast cells.

Localization is concentration dependent

Mao5 3.jpg

The researchers quantified localization of VM to phospholipid vesicles of varying diameters as well as at varying concentrations of VM. Researchers found a neutral or negative correlation between vesicle diameter and VM localization, depending on VM concentration, as in the leftmost plot. Researchers also found a positive correlation between VM concentration and VM localization, depending on the vesicle diameter; the correlation was stronger with smaller vesicles. The researchers proposed several possible explanations based on the data. Because VM was much smaller than the vesicles they localized to, it was unlikely that individual VM were capable of identifying curvature. Rather, there was probably a collective effect, in which the insertion of VM to vesicles indirectly recruited other VM. This recruitment was the result of there being an energetic or entropic pay-off from clustering VM. The last plot, on the far right, compares actual data to the predictions based on this collective effect model.