Protein flux across the membrane of single secretion granules.

We have applied, for the first time to our knowledge, X-ray microscopy to measure the mass of protein contained in single sub-cellular membrane-bound structures and to make high resolution, time-resolved observations on them. Using this method we have been able to follow the flux of protein out of secretion (zymogen) granules isolated from the acinar cells of the exocrine pancreas. The results provide direct visual and quantitative confirmation of the hypothesis that the membrane enclosing this object is permeable to its various contained proteins, although the mechanism remains unknown.