Particle-bound phytochrome: The nature of the interaction between pigment and particulate fractions

Particle-bound phytochrome from hypocotyl hooks of Cucurbita pepo L. seedlings is associated with either a "heavy" membrane fraction or a 31S ribonucleoprotein (RNP) component on sucrose gradients. Those amounts of phytochrome and RNP which co-pellet in response to added Mg(2+) are primarily localised in the 31S fraction. The phytochrome-RNP association itself, however, is not dependent on the added cation. This indicates that Mg(2+)-enhanced phytochrome pelletability results indirectly from aggregation of the RNP material. Phytochrome binds readily to this RNP fraction whether converted to the Pfr form in vivo or in vitro. Once bound, in either case, however, the pigment is not released by reconversion to Pr in vitro. Treatment of pellets with Triton X-100 causes most of the phytochrome to become sedimentable through 50% (w/w) sucrose, possibly indicative of pigment denaturation. Increasing the pH, in contrast, causes that phytochrome formerly located in the "heavy" membrane fraction to become associated with the 31S RNP component. High KCl concentrations dissociate the pigment from both "heavy" and 31S fractions, indicating the ionic nature of the interaction in both cases. These data can be accounted for by the electrostatic adsorption of phytochrome to ribosomal material, either ER-associated in the "heavy" fraction or "free" in the 31S fraction.Maize (Zea mays L.) exhibits a different pattern. Although Mg(2+) enhances the initial pelletability of both phytochrome and RNA the two components subsequently separate on Mg(2+)-free gradients.The data indicate that current interpretations of particle-bound phytochrome in terms of pigment-membrane interactions may need to be re-examined.