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 (VWFm) were investigated after expression in the 32D cell line. Normal proteolytic processing was observed for both proteins, however the processing of VWFm was much slower and a large portion was unprocessed. Results from subcellular fractionation and immunoelectron microscopy confirmed that a part of VWF, but not VWFm, was targeted to lysosome-related granules. Partial constitutive secretion was also observed for all forms of VWF and VWFm. 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. (C) 2004 Elsevier Inc. All rights reserved.