Isolation and Purification of the Photoreceptor Pigment Associated with Phototaxis in Dictyostelium discoideum

A pigment protein which undergoes a reversible light-induced absorbance change was purified about 2000-fold from cells of the phototactic cellular slime mold, Dictyostelium discoideum. The photoresponsive pigment was solubilized by sonication of the mitochondrial fraction, and purified by ammonium sulfate precipitation and centrifugation in sucrose gradients. The absorption spectrum of the photoresponsive pigment, which shows a strong band at 430 nm and two broad low extinction bands in the 550- to 590-nm region, is consistent with the action spectrum of phototaxis. Thus, this pigment appears to be the photoreceptor pigment for phototaxis in pseudoplasmodia of D. discoideum.