Pattern formation in cell-sized membranes




Shindell, Orrin Abraham

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The research presented in this dissertation follows in the tradition of experimental membrane biophysics. Our goal is to study the physical mechanisms underlying organization in the plasma membrane of living cells by using model systems. The central result from our experiments is that mixed-lipid membrane vesicles that are adhered by proteins to a solid-supported lipid membrane can dynamically form long-lived holes at the adhesion interface between the membranes. The first set of experiments we discuss exhibit the stable persistence of static patterns. The patterns are formed by adhering ternary-lipid vesicle membranes to a planar membrane supported on a solid, glass substrate \textit{via} biotin-avidin binding. The membrane and avidin are marked with spectrally distinct fluorescent dyes. We use fluorescence microscopy to acquire data. Adhesion causes half of adhered vesicles to form rough annular patterns with a central region that is devoid of membrane dye and protein binders. The peripheral region is dense in proteins and enriched in dye compared to the free, non-adhered portion of the same membrane. We measure the volume V and surface area A of adhered membranes. Using the measure 6[square root of pi]V/A[superscript 3/2] we find 0.84 for patterned and 0.98 for non-patterned membranes. Thus, adhered vesicles have two equilibrium states, one with annular patterns and one without, and the transition between them involves a loss of internal volume. Collectively our results suggest the annular patterns are holes. Finally, we report on a dynamic pattern that occurs in binary-lipid membranes adhered to a supported lipid bilayer. The pattern consists of finger-shaped holes that invade the protein-bound region. We show the characteristics of the fingers depend on the density [rho] of the protein binders in the adhered region: the width of static fingers [lambda] scales as [lambda] \sim\ [rho] and the rate of finger formation r, defined as the number of fingers that branch off from a boundary per unit time, scales as ln [r] \sim\ [rho]. Theoretically, we treat the formation of a finger as a thermally activated event occurring in a tense elastic film. The activation energy required to form a finger is approximately 3.5 kT, a biologically relevant energy scale.



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