Purification and characterization of Ca2+/calmodulin-dependent actin-binding proteins from squid retina

Ca2+/calmodulin (CaM)-dependent actin-binding proteins (CABPs) of 92, 105, 120 and 135 kDa were purified from squid retina. These proteins were eluted from the CaM affinity column in a Ca2+-dependent manner, and binding of the CABPs to F-actin was regulated by Ca2+/CaM. Electron microscopic observations employing the low-angle rotary shadowing technique showed the CABP molecules to have granular shapes similar to the granular proteins associated with actin filaments in squid rhabdomeral microvilli. We have previously reported that these actin filaments are fragmented upon exposure to light [(1988) J. Cell Biol. 106, 1151-1160]. Since the intracellular Ca2+ concentrations of the invertebrate retina are elevated during the light illumination, these results indicate that the CABPs are directly associated with the actin filament in the microvilli of the squid photoreceptors. We therefore suggest that the CABPs may regulate the light-induced structural changes of the microvillar cytoskeleton.