Mitogen-activated protein kinase (MAPK) cascades are involved in the regulation 
of cellular proliferation, differentiation, survival, apoptosis, as well as in 
inflammatory responses. Signal intensity and duration have been recognized as 
crucial parameters determining MAPK signaling output. Phosphatases play a 
particularly important role in this respect, by tightly controlling MAPK 
phosphorylation and activation. M3/6 (DUSP8) is a dual-specificity phosphatase 
implicated in the dephosphorylation and inactivation of JNK and, to a lesser 
extent, p38 MAPKs and is found in a complex with these kinases, along with other 
pathway components, held together by scaffold proteins. The JNK family consists 
of three genes, giving rise to at least ten different splice variants. Some 
functional differences between these gene products have been demonstrated, but 
the underlying molecular mechanisms and the roles of individual splice variants 
are still incompletely understood. We have investigated the interaction of M3/6 
with JNK isoforms, as well as scaffold proteins of the JNK interacting protein 
(JIP) family, in order to elucidate the contribution of M3/6 to the regulation 
of distinct JNK signaling modules. M3/6 exhibited stronger binding towards JNK1β 
and JNK2α isoforms and this was reflected in higher enzymatic activity towards 
JNK2α2 when compared to JNK1α1 in vitro. After activation of the pathway by 
exposure of cells to arsenite, the interaction of M3/6 with JNK1α and JNK3 was 
enhanced, whereas that with JNK1β or JNK2α decreased. The modulation of binding 
affinities was found to be independent of JNK-mediated M3/6 phosphorylation. 
Furthermore, arsenite treatment resulted in an inducible recruitment of M3/6 to 
JNK-interacting protein 3 (JIP3) scaffold complexes, while its interaction with 
JIP1 or JIP2 was constitutive. The presented data suggest an isoform-specific 
role for the M3/6 phosphatase and the dynamic targeting of M3/6 towards distinct 
JNK-containing signaling complexes.