Ane Landajuela Larma

Ane Landajuela Larma, Ph.D,



Little is known about mechanisms of membrane fission in bacteria despite their requirement for cytokinesis. The only known dedicated membrane fission machinery in bacteria, fission protein B (FisB), is expressed during sporulation in Bacillus subtilis and is required to release the developing spore into the mother cell cytoplasm. Recently we have characterized the requirements for FisB-mediated membrane fission. Take a look at the work here: http:// FisB forms mobile clusters of approximately 12 molecules that give way to an immobile cluster at the engulfment pole containing approximately 40 proteins at the time of membrane fission (Figure 1). Analysis of FisB mutants revealed that binding to acidic lipids and homo-oligomerization are both critical for targeting FisB to the engulfment pole and membrane fission. Experiments using artificial membranes and filamentous cells suggest that FisB does not have an intrinsic ability to sense or induce membrane curvature but can bridge membranes. Finally, modeling suggests that homo-oligomerization and trans-interactions with membranes are sufficient to explain FisB accumulation at the membrane neck that connects the engulfment membrane to the rest of the mother cell membrane during late stages of engulfment. Together, our results show that FisB is a robust and unusual membrane fission protein that relies on homo-oligomerization, lipid binding, and the unique membrane topology generated during engulfment for localization and membrane scission, but surprisingly, not on lipid microdomains, negative-curvature lipids, or curvature sensing.

Figure 1: Schematic of FisB dynamics. FisB freely moves around the engulfment membrane and other regions of the mother cell membrane, forming clusters of up to approximately 12 molecules. Cluster motions are independent of lipid microdomains, flotillins, the cell wall synthesis machinery, and voltage or pH gradients. About 40 copies of FisB accumulate at the membrane neck in an immobile cluster.

Recently we have gain insight into how FisB mediates membrane fission at the end of engulfment by revealing an novel aspect of chromosome translocation during spore development. You can take a look at the story here: http://http:// Sporulation initiates with an asymmetric division that generates a large mother cell and a smaller forespore that contains only 1/4 of its complete genome. As the mother cell membranes engulf the forespore, a DNA translocase pumps the rest of the chromosome into the small forespore compartment, inflating it due to increased turgor. When the engulfing membranes undergo fission, the forespore is released into the mother cell cytoplasm. Here we show that forespore inflation and FisB accumulation are both required for efficient membrane fission. We suggest that high membrane tension in the engulfment membrane caused by forespore inflation drives FisB-catalyzed membrane fission. Collectively our data indicate that DNA-translocation has a previously unappreciated second function in energizing FisB-mediated membrane fission under energy-limited conditions.

Figure 2: Model of how forespore inflation drives FisB-mediated membrane fission.
A. DNA translocation by the ATPase SpoIIIE inflates the forespore, stretching the thin peptidoglycan layer between two and smoothing membrane wrinkles (Lopez-Garrido et al.). Increased membrane tension in the engulfment membrane drives lipid flux through the neck between the engulfment and mother cell membranes, partially supplying lipids needed for forespore inflation.
B. FisB oligomerizes at the membrane neck, impeding membrane flow and causing the engulfment membrane to stretch further.
C. Increased membrane tension drives membrane scission.

I studied Biochemistry at the University of the Basque Country (UPV/EHU) and did my PhD at the BIOFISIKA Institute (UPV/EHU, CSIC) http:// . During this period I focused on the study of the mode of action of BCL-2 family proteins, in particular BH3-only proteins tBID, BIM and PUMA and how specific apoptosis-related lipids modulate these proteins. I followed a multidisciplinary approach focused on biochemical and biophysical techniques to examine the mechanisms by which these proteins regulate pro-apoptotic BAX/BAK functional activation. After that I stayed for two years in the same institute trying to understand the mechanism underlying membrane fusion mediated by the LC3 conjugation system, critical regulator for proper autophagosome biogenesis.

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