AN ARABIDOPSIS CDNA ENCODES AN APPARENT POLYPROTEIN OF 2 NONIDENTICALTHYLAKOID MEMBRANE-PROTEINS THAT ARE ASSOCIATED WITH PHOTOSYSTEM-II AND HOMOLOGOUS TO ALGAL YCF32 OPEN READING FRAMES

We have characterised an Arabidopsis thaliana cDNA homologous to the y cf32 open reading frames present in the Synechocystis genome and the p lastid genomes of several eukaryotic algae, The predicted protein is a lso homologous to a novel protein reported to be associated with photo system II, The protein is synthesised as a 23 kDa precursor with an N- terminal presequence that appears to be bipartite in structure, and th e protein is targeted into the thylakoid membrane of pea chloroplasts, Although the Ycf32 presequence contains an apparent signal peptide, w e find that this protein is not imported by either of the standard Sec -or Delta pH-dependent pathways, The mature protein is also unusual in two respects, First, there are two distinct, non-identical copies of typical single-span Ycf32 sequences in the Arabidopsis sequence, separ ated by an additional hydrophobic region, Secondly, the imported prote in runs as a doublet of 6 kDa and 7 kDa polypeptides whereas the matur e protein is predicted to be 14 kDa, We speculate that the protein und ergoes further maturation once inserted into the thylakoid membrane to yield two separate Ycf32-like polypeptides. (C) 1998 Federation of Eu ropean Biochemical Societies.