Structural insights into the inhibited states of the Mer receptor tyrosine kinase

The mammalian ortholog of the retroviral oncogene v-Eyk, and a receptor tyrosine kinase upstream of antiapoptotic and transforming signals, Met (MerTK) is a mediator of the phagocytic process, being involved in retinal and immune cell clearance and platelet aggregation. Met knockout mice are viable and are protected from epinephrine-induced pulmonary thromboembolism and ferric chloride-induced thrombosis. Met overexpression, on the other hand, is associated with numerous carcinomas. Although Met adaptor proteins and signaling pathways have been identified, it remains unclear how Met initiates phagocytosis. When bound to its nucleotide cofactor, the high-resolution structure of Met shows an autoinhibited alpha C-Glu-out conformation with insertion of an activation loop residue into the active site. Mer complexed with compound-52 (C52: 2-(2-hydroxyethylamino)-6-(3-chloroanilino)-9-isopropylpurine), a ligand identified from a focused library, retains its DFG-Asp-in and alpha C-Glu-out conformation, but acquires other conformational changes. The alpha C helix and DFGL region is closer to the hinge region and the ethanolamine moiety of C52 binds in the groove formed between Leu593 and Va1601 of the P-loop, causing a compression of the active site pocket. These conformational states reveal the mechanisms of autoinhibition, the pathophysiological basis of disease-causing mutations, and a platform for the development of chemical probes. (C) 2008 Elsevier Inc. All rights reserved.