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µ1A deficiency induces a profound increase in MPR300/IGF-II receptor internalization rate

¹ Zentrum für Biochemie und Molekulare Zellbiologie, Biochemie II, Universität Göttingen, Heinrich-Düker-Weg 12, D-37073 Göttingen, Germany
² School of Life Sciences, University of Dundee, Dundee DD1 5EH, Scotland, UK

*Author for correspondence (e-mail: pschu{at}gwdg.de)

Accepted September 13, 2001

The mannose-6-phosphate/IGF-II receptor MPR300 mediates sorting of lysosomal enzymes from the trans-Golgi network to endosomes and endocytosis of hormones, for example, of IGF-II. We analyzed transport of MPR300 in µ1A-adaptin-deficient fibroblasts, which lack a functional AP-1 clathrin adaptor complex. In µ1A-adaptin-deficient fibroblasts, the homologous MPR46 accumulates in endosomes due to a block in retrograde transport to the trans-Golgi network. The MPR300-mediated endocytosis is markedly enhanced. We demonstrate that the seven-fold increase in endocytosis is not associated with an increased steady-state concentration of receptors at the plasma membrane, but with an increased internalization rate of MPR300. Internalization of other receptors that are also endocytosed by AP-2 is not affected. More MPR300 receptors are found in clathrin-coated pits of the plasma membrane, whereas outside coated-areas, more MPR300 are concentrated in clusters and all intracellular receptors reside in endosomes, which are in equilibrium with the plasma membrane. Thus AP-1-mediated transport of MPR300 from endosomes to the TGN controls indirectly the recycling rate of the receptor between the plasma membrane and endosomes.

Key words: AP-1, clathrin, endocytosis, exocytosis, MPR300/IGF-II receptor

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