Lecithin:retinol acyltransferase-like proteins, also referred to as HRAS-like tumor suppressors, comprise a vertebrate subfamily of papain-like or NlpC/P60 thiol proteases that function as phospholipid-metabolizing enzymes. HRAS-like tumor suppressor 3, a representative member of this group, plays a key role in regulating triglyceride accumulation and energy expenditure in adipocytes and therefore constitutes a novel pharmacological target for treatment of metabolic disorders causing obesity. Here, we delineate a catalytic mechanism common to lecithin:retinol acyltransferase-like proteins and provide evidence for their alternative robust lipid-dependent acyltransferase enzymatic activity. We also determined high resolution crystal structures of HRAS-like tumor suppressor 2 and 3 to gain insight into their active site architecture. Based on this structural analysis, two conformational states of the catalytic Cys-113 were identified that differ in reactivity and thus could define the catalytic properties of these two proteins. Finally, these structures provide a model for the topology of these enzymes and allow identification of the protein-lipid bilayer interface. This study contributes to the enzymatic and structural understanding of HRAS-like tumor suppressor enzymes.
Background: The enzymology of aminophospholipid-processing enzymes is not well understood.
Results: Structures of HRAS-like tumor suppressors resemble those of thiol proteases with Cys-His-His catalytic triad.
Conclusion: An aminophospholipid acyl group first modifies the Cys residue of HRAS-like tumor suppressor enzymes before it is transferred onto a second substrate.
Significance: This study provides a structural basis for the enzymatic mechanism of HRAS-like tumor suppressors.
Enzyme Catalysis
Lipase
Lipids
Phosphatidylcholine
Structural Biology
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The atomic coordinates and structure factors (codes4DPZand4DOT) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).
This work was supported, in whole or in part, by National Institutes of Health Grants EY008061 (to K. P., M. G., and A. E. S.), EY009339 (to K. P., M. G., P. D. K, and A. E. S.), DK079221 (to W. B.), and EY019718 (to D. T. L.). This work was based upon research conducted at the Advanced Photon Source on the Northeastern Collaborative Access Team beamlines supported by National Institutes of Health Grants 5P41RR015301–10 from NCRR and Grant 8 P41 GM103403-10 from NIGMS.
