Peptidyl Prolyl Cis/Trans Isomerase NIMA Interacting Protein 4 (PIN4)

Sequence analysis predicted that the 131-amino acid PIN4 protein contains a PPIase domain preceded by a 40-amino acid glycine- and lysine-rich N-terminal sequence. PIN4, however, lacks a WW domain, a nuclear localization motif, and residues required for phosphoprotein interactions. Multiple-tissue Northern blot analysis detected variable expression of an approximately 1.0-kb PIN4 transcript in all tissues tested, with notably lower expression in neuronal tissue. Transmission electron microscopy demonstrated preferential localization of PIN4 in the mitochondrial matrix. Functional analysis failed to show PPIase activity, possibly due to proteolytic degradation or different substrate requirements.