PDK1 (3-phosphoinositide-dependent protein kinase-1) is a member of the AGC (cAMP-dependent, cGMP-dependent, protein kinase C) family of protein kinases, and has a key role in insulin and growth-factor signalling through phosphorylation and subsequent activation of a number of other AGC kinase family members, such as protein kinase B. The staurosporine derivative UCN-01 (7-hydroxystaurosporine) has been reported to be a potent inhibitor for PDK1, and is currently undergoing clinical trials for the treatment of cancer. Here, we report the crystal structures of staurosporine and UCN-01 in complex with the kinase domain of PDK1. We show that, although staurosporine and UCN-01 interact with the PDK1 active site in an overall similar manner, the UCN-01 7-hydroxy group, which is not present in staurosporine, generates direct and water-mediated hydrogen bonds with active-site residues. Inhibition data from UCN-01 tested against a panel of 29 different kinases show a different pattern of inhibition compared with staurosporine. We discuss how these differences in inhibition could be attributed to specific interactions with the additional 7-hydroxy group, as well as the size of the 7-hydroxy-group-binding pocket. This information could lead to opportunities for structure-based optimization of PDK1 inhibitors.

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