The 20 kDa apoprotein of the CP24 complex of photosystem II: An alternative model to study import and intra‐organellar routing of nuclear‐encoded thylakoid proteins

Abstract

SummaryThe 20 kDa polypeptide, the apoprotein of the chlorophyll a/b antenna complex CP24 associated with photosystem II, is a remote relative of light-harvesting complex (LHC) apoproteins and thus a member of the extended cab gene family. LHC apoproteins are polytopic integral components of the thylakoid membrane with probably three transmembrane segments which originate in nuclear genes and are made in the cytosol as precursors. They possess exclusively stromatargeting transit peptides for import into the organelle and integrate into the thylakoid membrane via uncleaved hydrophobic domains of the mature protein, The CP24 apoprotein displays intriguing structural differences to LHC apoproteins with a potential impact on the routing and targeting processes during biogenesis. In particular, it lacks a pronounced second hydrophobic segment in the mature polypeptide chain found in LHCPs, and carries a transit peptide that is reminiscent of thylakoid-targeting transit peptides. We have used in organello assays with isolated intact chloroplasts and the authentic precursor of the 20 kDa apoprotein from spinach, or appropriate chimaeric polypeptides consisting of a transit peptide and the mature part of various nuclear-encoded thylakoid proteins of known location and targeting epitopes, in order to resolve the characteristics of its targeting properties, as well as t o determine the contribution of the individual parts of the precursor molecule to its import and subsequent intra-organellar routing. Our experiments demonstrate that the transit peptide of the CP24 apoprotein is required only for the import of the protein into the organelle. All subsequent Steps, such as the integration of the protein into the thylakoid membrane, binding of chlorophyll, assembly into the cp24 complex and migration to the grana lamellae, still take place if the authentic transit peptide is re-

Differentially Expressed Forms of 1-L- -Inositol-1-Phosphate Synthase (EC) in

“…We also, surprisingly, identified a very large crossreacting protein or protein complex (approximately 80 kDa) with MI-1-P synthase activity, which localized to the chloroplast envelope and stroma. This protein could represent an active precursor, since most proteins targeted to the thylakoids are synthesized outside of the chloroplasts as larger molecular size precursors and processed to their mature size during transport (35)(36)(37). Routine Western analyses and ammonium sulfate fractionation cannot detect this protein.…”

A novel, bipartite transit peptide targets OEP75 to the outer membrane of the chloroplastic envelope.

“…The C-terminal domain subsequently directs the protein to the lumen. Most proteins destined for the chloroplastic inner envelope membrane (Li et al, 1992;Knight and Gray, 1995) and the thylakoid membrane (Hand et al, 1989;Cai et al, 1993) have transit peptides similar to those of precursors targeted to the stroma. Intraorganellar targeting information for these inner envelope membrane and thylakoid membrane precursor proteins apparently resides within the mature proteins.…”

Promoters from Genes for Plastid Proteins Possess Regions with Different Sensitivities toward Red and Blue Light

“…This could explain, at least in part, why each promoter functions optimally in its environment but responds differently, although not necessarily qualitatively differently, to regulatory pathways from other organisms. Deciphering the adaptation of regulatory sequences (and of sequences for import of the cytosol-synthesized proteins into the organelle; Wedel et al, 1992, Cai et al, 1993 to light-dependent signal pathways as well as of processes that led to controlled/coordinated expression of nuclear genes and of their plastid partner genes represents one of the principal challenges of modem cell biology.…”