Ubiquitination of the peroxisomal import receptor Pex5p is required for its recycling.

Pex5p, which is the import receptor for peroxisomal matrix proteins harboring a type I signal sequence (PTS1), is mono- and polyubiquitinated in Saccharomyces cerevisiae. We identified Pex5p as a molecular target for Pex4p-dependent monoubiquitination and demonstrated that either poly- or monoubiquitination of the receptor is required for the ATP-dependent release of the protein from the peroxisomal membrane to the cytosol as part of the receptor cycle. Therefore, the energy requirement of the peroxisomal import pathway has to be extended by a second ATP-dependent step, namely receptor monoubiquitination.