September 20, 2012

Once an egg finds its dance partner, the incredible changes that take place have fascinated biologists for centuries. These changes are kicked off immediately, and thanks to modern microscopy we get to see them up close. Today’s image is from a paper describing actin-mediated changes at the membrane of an egg shortly after fertilization.

Shortly after fertilization, actin assembly and dynamics drive changes in membrane physiology in the egg. One of these changes is an increase in efflux from the egg, or transport out of the egg, at the same time that short actin-based protrusions called microvilli are assembled around the egg. A recent paper investigates this link between efflux and microvilli after fertilization of sea urchin eggs. Whalen and colleagues found that a protein called ABCB1a translocates to the tips of microvilli shortly after fertilization. ABCB1a is an ABC transporter, a family transmembrane proteins that drive movement of various material into and out of many cell types. By using structured illumination microscopy (3D-SIM), Whalen and colleagues captured high-resolution images of actin filaments shortly after fertilization, and found movement of ABCB1a along growing actin filaments. Both efflux activity and movement of ABCB1a to microvilli tips were inhibited after actin polymerization was blocked. The images above show actin filaments (green) and ABCB1a (red) at different time points shortly after fertilization. By 60 minutes after fertilization, ABCB1a moved from below microvilli to the tips of the protrusions. 

Kristen Whalen, Adam M. Reitzel, & Amro Hamdoun (2012). Actin polymerization controls the activation of multidrug efflux at fertilization by translocation and fine-scale positioning of ABCB1 on microvilli Molecular Biology of the Cell, 23 (18), 3663-3672 DOI: 10.1091/mbc.E12-06-0438