Figure 4.

A significant fraction of ProL in INS cells becomes entrapped in long-term storage in the regulated secretory pathway. The stimulated (+) and unstimulated (−) secretions from pulselabeled INS cells were collected either during the first 4 h after labeling or after a chase of 20 h to first allow all newly synthesized ProL molecules to reach their final targets. At the conclusion of each set, both media and cell lysates were analyzed by immunoprecipitation with anti–cathepsin L. Two exposures of the media, differing fivefold, are shown in the upper two panels. Note the disappearance of labeled forms of mature L at later chase times, and the persistent stimulus-dependent secretion of the precursor. The positions of ProL and bands comprising mature cathepsin L (large bracket) are shown.