Sorting of Von Willebrand factor to lysosome-related granules of haematopoietic cells.

The aim of this work was to investigate sorting mechanisms of von Willebrand factor (VWF) when expressed in haematopoietic cells. The processing and sorting of both the wild-type VWF and a multimerization defective propeptide-mutant (VWF(m)) were investigated after expression in the 32D cell line. Normal proteolytic processing was observed for both proteins, however the processing of VWF(m) was much slower and a large portion was unprocessed. Results from subcellular fractionation and immunoelectron microscopy confirmed that a part of VWF, but not VWF(m), was targeted to lysosome-related granules. Partial constitutive secretion was also observed for all forms of VWF and VWF(m). Inhibition of acidification by chloroquine blocked VWF processing but allowed unprocessed pro-VWF targeting to dense organelles. In conclusion, our observations are consistent with VWF multimerization being of importance in cellular retention and targeting to lysosome-related organelles in haematopoietic cells, suggesting a role of protein aggregation for sorting in these cells.