Association of the NADPH : protochlorophyllide oxidoreductase (POR) with isolated etioplast inner membranes from wheat

Membrane association of NADPH:protochlorophyllide oxidoreductase (POR, EC: 1.6.99.1) with isolated prolamellar bodies (PLBs) and prothylakoids (PTs) f rom wheat etioplasts was investigated. in vitro-expressed radiolabelled POR , with or without transit peptide, was used to characterize membrane associ ation conditions. Proper association of POR with PLBs and PTs did not requi re the presequence, whereas NADPH and hydrolysable ATP were vital for the p rocess. After treating the membranes with thermolysin, sodium hydroxide or carbonate, a firm attachment of the POR protein to the membrane was found. Although the PLBs and PTs differ significantly in their relative amount of POR in vivo, no major differences in POR association capacity could be obse rved between the two membrane systems when exogenous NADPH was added, Exper iments run with only an endogenous NADPH source almost abolished associatio n of POR with both PLBs and PTs. In addition, POR protein carrying a mutati on in the putative nucleotide-binding site (ALA06) was unable to bind to th e inner membranes in the presence of NADPH, which further demonstrates that the co-factor is essential for proper membrane association. POR protein ca rrying a mutation in the substrate-binding site (ALA24) showed less binding to the membranes as compared to the wild type. The results presented here introduce studies of a novel area of protein-membrane interaction, namely t he association of proteins with a paracrystalline membrane structure, the P LB.