Processing and secretion of guanylate binding protein‐1 depend on inflammatory caspase activity

Human guanylate binding protein‐1 (GBP‐1) belongs to the family of large GTPases. The expression of GBP‐1 is inducible by inflammatory cytokines, and the protein is involved in inflammatory processes and host defence against cellular pathogens. GBP‐1 is the first GTPase which was described to be secreted by eukaryotic cells. Here, we report that precipitation of GBP‐1 with GMP‐agarose from cell culture supernatants co‐purified a 47‐kD fragment of GBP‐1 (p47‐GBP‐1) in addition to the 67‐kD full‐length form. MALDI‐TOF sequencing revealed that p47‐GBP‐1 corresponds to the C‐terminal helical part of GBP‐1 and lacks most of the globular GTPase domain. In silico analyses of protease target sites, together with cleavage experiments in vitro and in vivo, showed that p67‐GBP‐1 is cleaved by the inflammatory caspases 1 and 5, leading to the formation of p47‐GBP‐1. Furthermore, the secretion of p47‐GBP‐1 was found to occur via a non‐classical secretion pathway and to be dependent on caspase‐1 activity but independent of inflammasome activation. Finally, we showed that p47‐GBP‐1 represents the predominant form of secreted GBP‐1, both in cell culture supernatants and, in vivo, in the cerebrospinal fluid of patients with bacterial meningitis, indicating that it may represent the biologically active form of extracellular GBP‐1. These findings confirm the involvement of caspase‐1 in non‐classical secretion mechanisms and open novel perspectives for the extracellular function of secreted GBP‐1.