Treatment of leukemic cells with phorbol 12-myristate 13-acetate (PMA) induces a 
short-lived phosphorylation and activation of stress-activated protein kinase 
(SAPK) and cellular differentiation. To investigate whether the rapid 
deactivation of SAPK results from dephosphorylation by dual-specificity 
phosphatases (DSPs), we studied regulation of the DSP hVH5 and its murine 
orthologue M3/6 in K562 human leukemia cells. PMA treatment rapidly induced hVH5 
transcripts in these cells, and induced expression of M3/6 completely inhibited 
PMA-stimulated phosphorylation of SAPK, suggesting a feedback loop to control 
SAPK activity. Using both stable cell lines and transient transfection we 
demonstrate that activation of SAPK rapidly stimulated phosphorylation of M3/6. 
This phosphorylation did not regulate the half-life of total cellular M3/6. hVH5 
and M3/6 shares with all sequenced mammalian DSPs an amino acid motif, XILPXLXL, 
located approximately 80 amino acids from the active site. The hVH5-M3/6 
sequence, RILPHLYL, shares significant homology with the SAPK binding site of 
the c-Jun protein, called the delta domain. This motif was found to be important 
for DSP function, because deletion of RILPHLYL inhibits SAPK-mediated 
phosphorylation of M3/6, and deletion of this sequence or mutation of the LYL 
portion blocks the ability of this phosphatase to dephosphorylate SAPK.