Plasma membrane diffusion barriers in the precellularizing Drosophila embryo

Manos Mavrakis1, Richa Rikhy1, Bob Phair2, Jennifer Lippincott-Schwartz1. 1) Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892; 2) Integrative Bioinformatics Inc, Los Altos, CA 94024.

In this study we focus on the organization of the plasma membrane (PM) in the precellularizing Drosophila embryo. We have previously shown that each nuclei in the early embryo possess functionally compartmentalized secretory units in the absence of physical barriers. It is unknown whether proteins delivered to the PM are able to freely diffuse to all regions of the embryo, or whether the PM is compartmentalized over individual nuclei. To follow plasma membrane dynamics, we generated flies expressing fluorescently-tagged integral membrane proteins or fluorescently-tagged proteins associated with the inner leaflet of the plasma membrane, and performed photobleaching experiments. In FRAP experiments, both classes of membrane-associated proteins exhibited free lateral diffusion in the PM. FLIP assays showed, however, that the lateral diffusion of these proteins was restricted to regions of PM over individual nuclei. Finally, when a PM pool was optically pulse-chased in photoactivation experiments, proteins could freely diffuse in the embryo PM, but only within the PM region over individual nuclei and not across the PM over adjacent nuclei. Altogether our findings suggest the presence of a plasma membrane diffusion barrier that restricts lateral diffusion of proteins in the PM over multiple nuclei. We are currently investigating the nature of such diffusion barriers using a genetic approach, as well as by performing ultrastructural studies. Implications of this plasma membrane compartmentalization in the development of the early embryo are discussed.

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