Biophysical Society Thematic Meeting| Padova 2019

Quantitative Aspects of Membrane Fusion and Fission

Tuesday Speaker Abstracts

MULTISCALE MODELING OF PLANT VACUOLE FUSION IN GUARD CELLS: POSITIONING HOPS AS THE KEY REGULATOR OF STOMA MORPHOLOGY Belinda S. Akpa 1 ; David Flaherty 1 ; Natalie Clark 2 ; Aniket Antad 3 ; Rosangela Sozzani 2 ; Marcela Rojas-Pierce 2 ; 1 North Carolina State University, Molecular Biomedical Sciences, Raleigh, North Carolina, USA 2 North Carolina State University, Plant and Microbial Biology, Raleigh, North Carolina, USA 3 North Carolina State University, Electrical & Computer Engineering, Raleigh, North Carolina, USA Stomata are the pores on a leaf surface that regulate gas exchange. Each stoma is made of 2 guard cells whose movements regulate pore opening and thereby control CO2 fixation and water loss. Guard cell movements depend critically on the remodeling of cell vacuoles. These organelles have been observed to change morphology from a highly fragmented state to a fused state during stomata opening. The evolution of vacuole morphology requires a membrane fusion mechanism that responds rapidly to environmental signals, allowing plants to respond to diurnal cues or environmental stresses such as drought. With guard cells being both large and responsive to external signals, stomata represent a unique system in which to delineate mechanisms of membrane fusion and fission. Objective: To resolve a counter-intuitive observation regarding the role of HOPS in regulating vacuole morphology, we derived a quantitative model of vacuole fusion dynamics and used it to generate testable predictions about the dynamics of HOPS-SNARE interactions. Method: We derived our model from limited – and, initially, qualitative – data by integrating statistical inference and machine learning with quantitative fluorescence imaging and mechanistic modeling. The dynamic model predicted the evolution of vacuole morphology as it arises from intracellular signaling events that include: cytosol-to-membrane recruitment, chaperoned protein complexation, and complex disassembly. Results: We made specific predictions about the state of the biomolecular agents of fusion (e.g. HOPS, SNARE) prior to and during stoma opening. By constraining the model parameters to yield the emergent outcomes observed for stoma opening (as induced by two distinct signals), we proposed a dual role for HOPS and identified a stalled form of the SNARE complex that differs from phenomena reported in yeast. Conclusions: We predicted that HOPS has apparently contradictory actions at different points in the fusion signaling pathway, promoting the formation of SNARE complexes, but limiting their activity.

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