Hyperactive variants of p38α induce, whereas hyperactive variants of p38γ suppress, activating protein 1-mediated transcription

The p38 family of kinases is a subgroup of the mitogen-activated protein kinase family. It is composed of four isoforms and is involved in critical biological processes as well as in inflammatory diseases. The exact unique role of each p38 isoform in these processes is not understood well. To approach this question we have been developing intrinsically active variants of p38s. Recently we described a series of mutants of the human p38 alpha, which were spontaneously active as recombinant proteins purified from Escherichia coli cells. We show here that some of these mutants are spontaneously active in several mammalian cells in culture. The spontaneous activity of some mutants is higher than the activity of the fully activated wild type counterpart. We further produced mutants of the other p38 isoforms and found that p38 beta(D176A), p38 gamma(D179A), p38 delta(DI76A), and p38 delta(F324S) are spontaneously active in vivo. The active mutants are also spontaneously phosphorylated. To test whether the mutants actually fulfill downstream duties of p38 proteins, we tested their effect on activating protein 1 (AP-1)-mediated transcription. Active mutants of p38 alpha induced AP-1-driven reporter genes, as well as the c-jun and c-fos promoters. An active variant of p38 gamma suppressed AP-1-mediated transcription. When active variants of p38 alpha and p38-gamma were co-expressed, AP-1 activity was not induced, showing that p38 gamma is dominant over p38 alpha with respect to AP-1 activation. Thus, intrinsically active variants that are spontaneously active in vivo have been obtained for all p38 isoforms. These variants have disclosed different effects of each isoform on AP-1 activity.