Blessing or curse? Proteomics in granzyme research

Granzymes (gzms) are a group of serine proteases that play an important role in innate and adaptive immunity, blood coagulation, apoptosis, and inflammation, but are also connected to atherosclerosis, diabetes, cardiovascular and inflammatory lung diseases, cancer, and sepsis. Humans have five gzms (gzms A, B, H, K, and M), which differ in their substrate specificity. It is widely accepted that they are delivered from cytotoxic lymphocytes via perforin into the cytoplasm of target cells where they initiate cell death, modulate cytokine signaling, or inactivate pathogen proteins. However, more recent evidence indicates gzms also act extracellularly in noncytotoxic processes. Proteomic approaches are directed at mapping gzm cleavage specificity, identifying substrates, and unraveling the (patho-) physiological role of these proteases. These studies have refined our understanding of gzm species specificity, and collectively uncovered an enormous number of new substrates. However, with the exception of a very few human gzmB substrates supported by independent data (Bid, DNA-PK, PARP, ICAD, and procaspase 7), it is presently unclear which are physiologically relevant. This review aims to summarize and analyze the different proteomic approaches used and discuss both their convincing and controversial outcomes.