CLATHRIN-COATED PIT-MEDIATED RECEPTOR INTERNALIZATION - ROLE OF INTERNALIZATION SIGNALS AND RECEPTOR MOBILITY

Most signals controlling receptor-mediated endocytosis have been ident ified by alteration of sequences present in receptors normally interna lized via clathrin-coated pits. In the present work we have reconsider ed the factors that control internalization the other way around: i.e. by introducing potential internalization sequences in complement rece ptor 1 (CR1) which does not preferentially associate with clathrin-coa ted pits. The analysis of the internalization efficiency of NPxY relat ed motifs generated by substituting His2010 and/or Glu2015 by either P he or Tyr indicates that FxNPxY is the stronger promoter of endocytosi s and that the signal efficiency depends on the presence of aromatic r esidues (including a tyrosine) at both ends of the -xNPx- motif. Moreo ver, CR1-tyr (substitution of Glu2015 for Tyr) internalization was sup erposable to that of a receptor composed of the extracellular and tran smembrane domains of CR1 fused to the intracytoplasmic tail of the low density lipoprotein (LDL) receptor (including the FxNPxY motif) (CR1- LDL). When analyzed by fluorescence recovery after photo-bleaching, th e surface mobility of CR1-LDL was decreased as compared with that of e ither CR1-tyr or CR1-wt, despite a similar association with clathrin-c oated pits. The role of receptor mobility in internalization was confi rmed by the observation that CR1-wt. with a deletion of the cytoplasmi c tail, was more mobile and more efficiently internalized than CR1-wt.