Purification of Recombinant Human cPLA2γ and Identification of C-Terminal Farnesylation, Proteolytic Processing, and Carboxymethylation by MALDI-TOF-TOF Analysis

Abstract

Cytosolic phospholipase A₂γ (cPLA₂γ) is a calcium-independent, membrane-associated phospholipase A₂ that possesses a C-terminal prenylation motif (-CCLA) whose covalent structure cannot be deduced from the primary sequence alone. Accordingly, we overexpressed human cPLA₂γ containing an N-terminal His tag ((His)₆cPLA₂γ) in Sf9 cells and quantitatively solubilized and purified the enzyme by sequential immobilized metal affinity and Mono Q column chromatographies. The final preparation appeared as a single 61 kDa band after SDS−PAGE/silver-staining, possessed high lysophospholipase activity (50 μmol min^-1 mg^-1), and was inhibited by, but did not hydrolyze, palmitoyl-CoA. Radiolabeling of recombinant human cPLA₂γ with [³H]-mevalonolactone in the absence of statins and subsequent cleavage of prenyl groups with Raney nickel revealed that the enzyme is only farnesylated and is not geranylgeranylated. Analysis of CNBr-digested cPLA₂γ by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI/TOF-TOF) mass spectrometry demonstrated the presence of a farnesyl moiety at Cys-538, cleavage of the Cys⁵³⁸−Cys⁵³⁹ bond, and carboxymethylation of the resultant C-terminal prenylated cysteine. Collectively, these results describe the solubilization and purification of recombinant cPLA₂γ to homogeneity and identify cPLA₂γ as a farnesylated protein that undergoes at least three sequential posttranslational modifications that likely facilitate its targeting and interactions with its membrane substrates.