One of the most studied secreted phospholipases A 2 (sPLA 2 ), the group IIA sPLA 2 , is found at high levels in inflammatory fluids of patients with autoimmune diseases. A characteristic of group IIA sPLA 2 is its preference for negatively charged phospholipids, which become exposed on the extracellular leaflet of apoptotic cell membranes. We recently showed that low molecular weight heparan sulfate proteoglycans (HSPGs) and uncharacterized detergent-insoluble binding site(s) contribute to the enhanced binding of human group IIA PLA 2 (hGIIA) to apoptotic human T cells. Using matrixassisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry we now identify vimentin as the major HSPG-independent binding protein of hGIIA on apoptotic primary T lymphocytes. Vimentin is partially exposed on the surface of apoptotic T cells and binds hGIIA via its rod domain in a calciumindependent manner. Studies with hGIIA mutants showed that specific motifs in the interfacial binding surface are in-
IntroductionPhospholipases A 2 (PLA 2 ) hydrolyze membrane phospholipids to produce free fatty acids and lysophospholipids. 1,2 PLA 2 provide precursors for the biosynthesis of proinflammatory lipid mediators such as prostaglandins, leukotrienes, and lipoxins when the hydrolyzed fatty acid is arachidonic acid (AA) and platelet-activating factor when the lysophospholipid is 1-alkyl-2-lyso-phosphatidylcholine. Several high molecular weight intracellular mammalian PLA 2 and a family of 14-to 19-kDa PLA 2 , referred to as secretory PLA 2 (sPLA 2 ), have been identified. 2,3 Mammalian sPLA 2 are classified into 10 different groups. 3,4 sPLA 2 can interact with external cell membranes through direct binding to phospholipids or specific cell surface receptors. 3,5,6 The M-type receptor present on skeletal muscle 7 and human neutrophils 8 belongs to the superfamily of C-type lectins and binds with high affinity to the snake venom sPLA 2 OS 1 and OS 2 , pancreatic group IB PLA 2 , and rabbit and mouse group IIA PLA 2 but not to bee venom group III PLA 2 and very weakly to human group IIA PLA 2 (hGIIA). 7,[9][10][11][12][13] The N-type receptors identified in rat brain have high affinity for OS 2 and bee venom group III PLA 2 . 14 Venom and mammalian sPLA 2 also bind to a number of cell soluble proteins, such as pentraxins, 15 reticulocalbins, 16 calmodulin, 17 heparin, and to the serum coagulation factor Xa. 18 Several mammalian sPLA 2 such as group II and V enzymes also bind to heparan sulfate proteoglycans (HSPGs) like glypican-1 19 and decorin. 20 hGIIA is found in high concentrations in the circulation of patients with acute pancreatitis and certain cancers and in the synovial fluid from patients with rheumatoid arthritis. 1,2,[21][22][23] hGIIA has been proposed to release AA for the generation of prostaglandins in certain conditions. 19,[24][25][26][27][28][29][30][31] The binding of hGIIA to HSPGs is required for exogenous hGIIA-catalyzed AA release from apoptotic human T cells and from HEK-293 cells overexpressin...