Protein quaternary structure and expression levels contribute to peroxisomal-targeting-sequence-1-mediated peroxisomal import of human soluble epoxide hydrolase.
The peroxisomal targeting sequence 1 (PTS1) is a consensus tripeptide 1 (S/C/A)(K/R/H)(L/M) that is found at the C-terminus of most peroxisomal proteins. However, the only known mammalian protein containing a terminal methionine PTS1 (SKM), human soluble epoxide hydrolase (hsEH), shows both peroxisomal and cytosolic localizations in vivo. Mechanisms regulating the subcellular localization of hsEH thus remain unclear. Here we utilized green fluorescent protein-hsEH fusion constructs to study the peroxisomal targeting of hsEH in transiently and stably transfected Chinese hamster ovary cells. Our results suggest that the peroxisomal import of hsEH is regulated by three factors. First, we show that SKM is required, but not sufficient, for peroxisomal import. Second, by manipulating protein expression levels, we show that SKM mediates peroxisomal import of wild-type hsEH only when expression levels are high. Third, we show that amino acid modifications that decrease subunit oligomerization and presumably enhance accessibility of the SKM motif confer peroxisomal targeting even at low protein expression levels. We conclude that, in hsEH, SKM is a necessary but inefficient and context-dependent PTS1. Peroxisomal import occurs when expression levels are high or when the SKM motif is accessible. These results provide a mechanistic basis for understanding the cell-specific and tissue-specific localization of hsEH in vivo.
Luo
Beibei
B
Department of Pharmaceutical Sciences, University of Connecticut, Storrs, CT 06269, USA.
Norris
Carol
C
Bolstad
Erin S D
ES
Knecht
David A
DA
Grant
David F
DF
D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D004851:Epoxide Hydrolases / Q000737:chemistry* / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D050505:Mutant Proteins / Q000378:metabolism; D020675:Peroxisomes / Q000201:enzymology*; D011485:Protein Binding; D021382:Protein Sorting Signals; D020836:Protein Structure, Quaternary; D021381:Protein Transport; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012995:Solubility; D013329:Structure-Activity Relationship
D050505:Mutant Proteins; D021382:Protein Sorting Signals; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D004851:Epoxide Hydrolases