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 p38alpha, 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 p38beta(D176A), 
p38gamma(D179A), p38delta(D176A), and p38delta(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 p38alpha induced AP-1-driven reporter genes, as well as the c-jun and 
c-fos promoters. An active variant of p38gamma suppressed AP-1-mediated 
transcription. When active variants of p38alpha and p38gamma were co-expressed, 
AP-1 activity was not induced, showing that p38gamma is dominant over p38alpha 
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.