70-kDa peroxisomal membrane protein related protein (P70R/ABCD4) is a member of ATP-binding cassette (ABC) protein subfamily D. ABC subfamily D proteins are also known as peroxisomal ABC proteins. Therefore, P70R is thought to be a peroxisomal membrane protein. However, the subcellular localization of P70R is not extensively investigated. In this study, we transiently expressed P70R in fusion with HA (P70R-HA) in CHO cells and examined subcellular localization by immunofluorescence. Surprisingly, P70R-HA was localized to the endoplasmic reticulum (ER), not to peroxisomes. To examine the ER-targeting property of P70R, we expressed various NH2-terminal deletion constructs of P70R. Among the NH2-terminal deletion constructs, mutant proteins starting with hydrophobic transmembrane segment (TMS) were localized to ER, but the ones containing the NH2-terminal hydrophilic cytosolic domain were not. ABC subfamily D proteins destined for peroxisomes have NH2-terminal hydrophilic region adjacent to TMS1. However, only P70R lacks the region and is translated with NH2-terminal hydrophobic TMS1. Furthermore, attachment of the NH2-terminal hydrophilic domain to the NH2-terminus of P70R excluded P70R from the ER-targeting pathway. These data suggest that P70R resides in the ER but not the peroxisomal membranes, and the hydrophobic property of NH2-terminal region determines the subcellular localization of ABC subfamily D proteins.
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