Identification of PEX5p-related novel peroxisome-targeting signal 1 (PTS1)-binding proteins in mammals

Based on peroxin protein 5 (Pex5p) homology searches in the expressed seque nce tag database and sequencing of large full-length cDNA inserts, three no vel and related human cDNAs were identified. The brain-derived cDNAs coded for two related proteins that differ only slightly at their N-terminus, and exhibit 39.8% identity to human PEX5p. The shorter liver-derived cDNA code d for the C-terminal tetratricopeptide repeat-containing domain of the brai n cDNA-encoded proteins. Since these three proteins specifically bind to va rious C-terminal peroxisome-targeting signals in a manner indistinguishable from Pex5p and effectively compete with Pex5p in an in vitro peroxisome-ta rgeting signal 1 (PTS1)-binding assay, we refer to them as 'Pex5p-related p roteins' (Pex5Rp). In contrast to Pex5p, however, human PEX5Rp did not bind to Pex14p or to the RING finger motif of Pex12p, and could not restore PTS I protein import in Pex5(-/-) mouse fibroblasts. Immunofluorescence analysi s of epitope-tagged PEX5Rp in Chinese hamster ovary cells suggested an excl usively cytosolic localization. Northern-blot analysis showed that the PEX5 R gene, which is localized to chromosome 3q26.2-3q27, is expressed preferen tially in brain. Mouse PEX5Rp was also delineated. In addition.. experiment al evidence established that the closest-related yeast homologue, YMR018wp, did not bind PTSI. Based on its subcellular localization and binding prope rties, Pex5Rp may function as a regulator in an early step of the PTSI prot ein import process.