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

The 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 stroma-targeting 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 to 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 replaced by a targeting signal of a nuclear-encoded stromal protein.