EDITOR'S CHOICE IN BIOPHYSICS
LET THERE BE COLOR: Iridocyte membranes consist of lots of tightly packed parallel folds, creating numerous extracellular channels between lamellae containing reflectin proteins (1). The neurotransmitter ACh activates a cascade of signals in the cell that results in reflectin condensation, making the membrane reflective and iridescent (2). Ions released by the condensation of reflectins cross the membrane into the extracellular space, which in turn drives the expulsion of water from lamellae (3). This shrinks the lamellae, changing the thickness of and spacing between the membrane’s deep grooves to reflect light in a variable way that produces colors (4).© SCOTT LEIGHTONThe paper
D.G. DeMartini et al., “Membrane invaginations facilitate reversible water flux driving tunable iridescence in a dynamic biophotonic system,” PNAS, 110:2552-56, 2013.
From hummingbirds to herring, a dazzling range of animals boast structural color—the brilliant iridescent hues that result not from pigment, but from light reflecting off microscale structures in feathers or skin cells. (See “Color from Structure,” The Scientist, February 2013.) However, only a select few cephalopods can rapidly fine-tune their iridescent colors for communication or camouflage. They do so by tweaking the reflective properties of an array of deep grooves in the plasma membranes of specialized cells called iridocytes. But exactly how the membrane is manipulated, and how those changes produce the whole spectrum of colors, has been unclear.
In 2010, Daniel Morse of the University of California, Santa Barbara, and colleagues showed that when the neurotransmitter acetylcholine (ACh) binds to a receptor on iridocytes it triggers a cascade of signals that results in the phosphorylation and consequent condensation of reflective proteins called reflectins. This year, Morse and his team took an even closer look at how the iridocytes respond to activation by ACh.
First, they isolated iridocytes from squid (Doryteuthis opalescens) and examined the cells’ surface using a high-resolution scanning electron microscope. To their surprise, they saw that the grooves on the plasma membrane are made up of a series of tightly packed parallel invaginations that penetrate deep into the cell like pleats in a drape. Dissecting away parts ...
