Fig. 3. The C2A of synaptotagmin VII is internalized in a cell-type-independent and tryptophan-independent manner. Using the previously described internalization assay, the CD4-C2A constructs of synaptotagmin I (residues 95-265, C2A 1) and synaptotagmin VII (residues 98-260, C2A 7) were tested. PC12 cells (A) and CHO cells (B) stably expressing the constructs were both analyzed by this method. (C) A point mutant changing the tryptophan residue of C2A 7's WKXL (W253A) to an alanine did not affect the strong endocytosis of C2A 7 in PC12 cells.