Mapping the BCSC-1 interactome in breast cancer

BCSC-1 (breast cancer suppressor candidate-1), the official gene symbol VWA5A (von Willebrand factor A domain containing 5 A), is downregulated in a variety of cancer types including breast cancer. The role of BCSC-1 in tumorigenesis and the underlying mechanisms are poorly understood. One of the ways to predict a spectrum of functions of an unknown protein is to identify the protein-protein interaction network involving it. Flag-pLV-Neo-BCSC1 or the empty vector were stably transfected into breast cancer cells MCF-7. The levels of BCSC1 expression were identified by Western blotting. Co-immunoprecipitation assay coupled with liquid chromatography with tandem mass spectrometry (Co-IP-MS) and bioinformatic analysis were done to study protein-protein interactions. A subset of interacting proteins was validated by immunofluorescence (IF). IF was also conducted to determine the cell migration of BCSC-1. The pEGFP-C3 plasmids with full-length and truncated BCSC-1 were transiently transfected into breast cancer cells to identify domains of BCSC-1 guiding the subcellular location. 341 interacting partners of BCSC-1 were significantly enriched in Flag-BCSC-1 against negative control. These interactors included the lysosome and the proteasome related to proteostatic pathways, the peroxisome involved in lipid metabolism, endocytosis components in the vesicle transport pathway, and the regulation of actin cytoskeleton. STAT1, STAT3, and CDC42, identified among the interacting partners, co-localized with BCSC-1. In addition, BCSC-1 distributed to the leading edge of migrating breast cancer cells. VIT (vault protein inter-α-trypsin) domain of BCSC-1 is required in the regulation of cell migration. BCSC-1 might exert diverse functions through protein-protein interactions.