S. Reinbothe et al., A PLASTID ENZYME ARRESTED IN THE STEP OF PRECURSOR TRANSLOCATION IN-VIVO, Proceedings of the National Academy of Sciences of the United Statesof America, 93(21), 1996, pp. 12026-12030
The key enzyme of chlorophyll biosynthesis in higher plants, NADPH:pro
tochlorophyllide (Pchlide) oxidoreductase (FOR, EC 1.3.1.33), accumula
tes in its precursor form (pPORA) in barley. pPORA is bound to the chl
oroplasts and is able to interact with the enzyme's substrate, Pchlide
, at both the cytosolic as well as the stromal side of the plastid env
elope. The interaction with intraplastidic Pchlide, formed in ATF-cont
aining chloroplasts upon feeding with delta-aminole-vulinic acid, driv
es vectorial translocation of pPORA across the plastid envelope membra
nes. In contrast, exogenously applied Pchlide causes the release of th
e envelope-hound precursor protein to the cytosol. Both processes comp
ete with each other if intra- and extraplastidic Pchlide are applied s
imultaneously. A cytosolic heat shock cognate protein of M(r) 70,000 p
resent in wheat germ and barley leaf protein extracts appears to preve
nt the release of the pPORA to the cytosol in vivo, however.