A role for lysosomes in the turnover of human iron regulatory protein 2

The two mammalian iron regulatory proteins, IRP1 and IRP2, are post-transcriptional regulators of cellular iron homeostasis. These cytosolic RNA-binding proteins control the synthesis of proteins involved in storage, transport, and utilization of iron. Whereas IRP1 levels remain nearly constant, IRP2 is rapidly degraded by the proteasome in iron-replete cells. In non iron-loaded H1299 human lung cancer cells, the decay of transfected hemagglutinin-tagged IRP2 was significantly antagonized by addition of not only proteasomal, but also lysosomal inhibitors. Similar results were obtained with IRP2(-Ins5), a molecular form lacking the specific IRP2 domain of 73 amino acids that is absent from IRP1. These data uncover an alternative, iron independent, mechanism of IRP2 degradation via the lysosomal pathway. Transfected IRP1 decayed slowly over several days and, in contrast to IRP2, was not further stabilized by proteasomal or lysosomal inhibitors. Experiments with an IRP1/IRP2 hybrid molecule and with IRP2 variants indicated that proteins lacking the C-terminus of IRP2 were insensitive to lysosomal inhibitors. Together with previous data obtained in the presence of iron excess, these results show that the parallel degradation pathways through lysosomes and the proteasome that are active on IRP2 under normal growth conditions are preferentially shifted to the proteasome when iron becomes plentiful.