The Von Willebrand Factor Propeptide (VWFpp) Traffics an Unrelated Protein to Storage

The von Willebrand factor (VWF) propeptide (VWFpp) is critical for the targeting of VWF multimers to storage granules. VWFpp alone efficiently navigates the storage pathway in AtT-20 and endothelial cells and chaperones mature VWF multimers to storage granules when the two proteins are expressed in cis or in trans. To further define the role of VWFpp in granular sorting, we examined its ability to sort an unrelated protein, C3&agr; into the regulated secretory pathway. Chimeric constructs of VWFpp and the &agr;-chain of C3 were developed. The C3&agr; protein expressed alone did not sort to granules in AtT-20 cells. The trans expression of C3&agr; and VWFpp resulted in granular storage of VWFpp but no corresponding storage of C3&agr;. When C3&agr; is expressed as a single chain molecule with VWFpp that was rendered uncleavable by furin, C3&agr; is re-routed to storage and is colocalized with VWFpp. The uncleavable protein was expressed in bovine aortic endothelial cells where it sorted to Weibel-Palade bodies, colocalized with bovine VWF, and was released when agonist stimulated. We now demonstrate that VWFpp re-routes a constitutively secreted protein to the regulated storage pathway. Furthermore, our studies suggest that the VWFpp storage signal is contained within amino acids 201 to 741.