mprs are not found in the lysosomes; they cycle mainly between the "trans"-golgi network and endosomes. the ci-mpr is also present on the cell surface. around 10-20% of the ci-mpr can be found at the cell membrane. its function here is to capture any mannose 6-phosphate tagged enzymes that have accidentally entered the secretory pathway. once it binds to a lysosomal enzyme the receptor becomes internalised rapidly. internalisation is mediated by a sorting signal in its cytoplasmic tail – a yskv motif. this ensures that all harmful lysosomal enzymes will be targeted to the lysosome.