TI - Phosphorylation of Ser28 in histone H3 mediated by mixed lineage kinase -like mitogen-activated protein triple kinase alpha . AB - The mitogen -activated protein kinase cascades elicit modification of chromatin proteins such as histone H3 by phosphorylation concomitant with gene activation . Here , we demonstrate for the first time that the mixed lineage kinase -like mitogen-activated protein triple kinase ( MLTK ) -alpha phosphorylates histone H3 at Ser28 . MLTK-alpha but neither a kinase -negative mutant of MLTK-alpha nor MLTK-beta interacted with and phosphorylated histone H3 in vivo and in vitro . When overexpressed in 293T or JB6 Cl41 cells , MLTK-alpha phosphorylated histone H3 at Ser28 but not at Ser10 . The interaction between MLTK-alpha and histone H3 was enhanced by stimulation with ultraviolet B light ( UVB ) or epidermal growth factor ( EGF ) , which resulted in the accumulation of MLTK-alpha in the nucleus . UVB - or EGF -induced phosphorylation of histone H3 at Ser28 was not affected by PD 98059 , a MEK inhibitor , or SB 202190 , a p38 kinase inhibitor , in MLTK-alpha-overexpressing JB6 Cl41 cells . Significantly , UVB - or EGF -induced phosphorylation of histone H3 at Ser28 was blocked by small interfering RNA of MLTK-alpha . The inhibition of histone H3 phosphorylation at Ser28 in the MLTK-alpha knock-down JB6 Cl41 cells was not due to a defect in mitogen - and stress-activated protein kinase 1 or 90-kDa ribosomal S6 kinase ( p90RSK ) activity . In summary , these results illustrate that MLTK-alpha plays a key role in the UVB - and EGF -induced phosphorylation of histone H3 at Ser28 , suggesting that MLTK-alpha might be a new histone H3 kinase at the level of mitogen -activated protein kinase kinase kinases .