A protochlorophyllide-dependent protein import complex of chloroplasts

Abstract

Chloroplasts synthesize an abundance of different tetrapyrrole compounds. Among them are chlorophyll and its precursor protochlorophyllide, which accumulate in light- and dark-grown plants, respectively. Protochlorophyllide is bound to two NADPH:Pchlide oxidoreductases (POR), termed PORA and PORB, which in angiosperms form higher molecular weight complexes in the prolamellar body of etioplasts. Both POR proteins are nuclear gene products which must be imported from the cytosol.. Import of the precursor of the PORA requires Pchlide and is due to the operation of a second, substrate-dependent protein import site in the plastid envelope. Here we report the isolation of components of the Pchlide-dependent import (PIM) complex and show that it is composed of at least 12 different polypeptides. Among them is a Pchlide a oxygenase which provides Pchlide b as the import substrate of the pPORA, and a tyrosine amino transferase presumably involved in conferring photoprotection onto the PIM complex. Two other constituents were found to be related to previously characterized outer envelope membrane proteins: a 33K protein to TOC33 which is a GTP binding protein, and a 16K protein to a voltage-dependent, amino acid-sective channel protein in the outer plastid envelope.