Exploring the Landscape of Ectodomain Shedding by Quantitative Protein Terminomics

Ectodomain shedding is a proteolytic process that regulates the levels and functions of membrane proteins. Dysregulated shedding is linked to severe diseases, including cancer and Alzheimer9s disease. However, the exact cleavage sites of shedding substrates remain largely unknown. Here, we explore the landscape of ectodomain shedding by generating large-scale, cell-type-specific maps of shedding cleavage sites. By means of N- and C-terminal peptide enrichment and quantitative mass spectrometry, we quantified protein termini in the culture media of 10 human cell lines and identified 411 cleavage sites on the ectodomain of 132 membrane proteins whose proteolytic terminal fragments are downregulated in the presence of a broad-spectrum metalloprotease inhibitor. A major fraction of the presented cleavage sites was identified in a cell-type-specific manner, and mapped onto receptors, cell adhesion molecules, and protein kinases and phosphatases. We confidently identified 86 cleavage sites as metalloprotease substrates by means of knowledge-based scoring.