Regulation of cytosolic phospholipase A(2) in a new perspective: Recruitment of active monomers from an inactive clustered pool

cPLA(2) plays a key role in many signal transduction cascades by hydrolyzin g arachidonic acid from membrane phospholipids. Tight control of cPLA(2) ac tivity by a number of regulatory mechanisms is essential to its cellular fu nction. We recently described the localization of cPLA(2) in clusters in fi broblasts and now propose that these clusters reflect a localized inactive pool from which active monomers can be recruited to keep cPLA(2) activity u nder control on the subcellular level. Using an electron microscopic in vit ro approach, we show that cPLA(2) monomers, but not the clusters, bind to m embranes in a Ca2+-dependent manner. This binding is accompanied by hydroly tic activity. The present data combined with our previous observation of a relative abundance of clusters over monomers in fixed fibroblasts [Bunt, G, , de Wit, J., van den Bosch, H., Verkleij, A., and Boonstra, J. (1997) J. C ell Sci. 110, 2449-2459] gives rise to a concept of cPLA(2) regulation in w hich small amounts of active monomers are recruited to fulfill their functi on upon stimulation. This is in contrast to processes described for inflamm atory cells, where a substantial part of the cytoplasmically localized cPLA (2) translocates to the perinuclear region upon stimulation to become activ e. Small-scale regulation of cPLA(2) by the proposed cluster-monomer cycle allows local and strictly confined control of cPLA(2) activity, apparently necessary for its cellular role in fibroblasts.