title: Unusual characteristics of amino-terminal and hydrophobic domains in nuclear-encoded thylakoid signal peptides.
abstract: Thylakoid transfer signals carry information specifying translocation by either a Sec- or delta pH-dependent protein translocator in the chloroplast thylakoid membrane, yet all resemble classical signal peptides in overall structural terms. Comparison of known transfer signals reveals two differences: (a) signals for the delta pH-driven system invariably contain a critical twin-arginine (Arg-Arg) motif prior to the hydrophobic (H) domain, whereas known Sec-dependent signals contain lysine, and (b) the H-domains of Sec-dependent signals are generally longer. Previous work has shown that a twin-Arg motif before the H-domain is critical for targeting by the delta pH-dependent pathway; in this report we show that the charge characteristics of this region are not important for sorting by the Sec pathway. Twin-Lys, twin-Arg or single Arg are all acceptable to the Sec system, although single Lys/Arg is preferred. The single Lys in pre-plastocyanin can even be replaced by an uncharged residue without apparent effect. We have also generated a pre-plastocyanin mutant containing an H-domain which, in terms of hydropathy profile, is identical to that of a delta pH-dependent protein. This mutant is also transported efficiently by the Sec system, demonstrating that hydrophobicity per se is not a key sorting determinant. However, the characteristics of the H-domain may be important in avoiding a different form of mis-targeting: to the endoplasmic reticulum. Thylakoid signal peptides have undergone substantial structural changes during the evolution of the chloroplast from endosymbiotic cyanobacterium: plastid-encoded and cyanobacterial signals contain H-domains that are highly hydrophobic and enriched in Leu and aromatic residues, whereas nuclear-encoded counterparts are Ala-rich and far less hydrophobic. We speculate that this trend may reflect a need to avoid mistargeting through recognition by cytosolic signal recognition particle, which preferentially interacts with more hydrophobic signal peptides.
authors.affiliation: Department of Biological Sciences, University of Warwick, Coventry, UK.
authors.forename: S
authors.initials: S
authors.lastname: Brink
authors.forename: E G
authors.initials: EG
authors.lastname: Bogsch
authors.forename: A
authors.initials: A
authors.lastname: Mant
authors.forename: C
authors.initials: C
authors.lastname: Robinson
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D001693:Biological Transport, Active; D002467:Cell Nucleus / Q000378:metabolism; D000460:Chlorophyta / Q000737:chemistry; D002736:Chloroplasts / Q000737:chemistry; D000458:Cyanobacteria / Q000737:chemistry; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D010449:Peptide Mapping; D010940:Plant Proteins / Q000737:chemistry*; D018087:Plastids / Q000737:chemistry; D010970:Plastocyanin / Q000737:chemistry; D021382:Protein Sorting Signals / Q000737:chemistry*; D013329:Structure-Activity Relationship
chemical_list: D001426:Bacterial Proteins; D010940:Plant Proteins; D021382:Protein Sorting Signals; D010970:Plastocyanin