TI - Phosphorylation of human keratin 8 in vivo at conserved head domain serine 23 and at epidermal growth factor -stimulated tail domain serine 431 . AB - Dynamic phosphorylation is one mechanism that regulates the more than 20 keratin type I and II intermediate filament proteins in epithelial cells . The major type II keratin in "simple type" glandular epithelia is keratin 8 ( K8 ) . We used biochemical and mutational approaches to localize two major in vivo phosphorylation sites of human K8 to the head ( Ser-23 ) and tail (Ser-431) domains . Since Ser-23 of K8 is highly conserved among all type II keratins , we also examined if the corresponding Ser-59 in stratified epithelial keratin 6e is phosphorylated . Mutation of K6e Ser-59 abolished its phosphorylation in 32PO4-labeled baby hamster kidney cell transfectants . With regard to K8 phosphorylation at Ser-431 , it increases dramatically upon stimulation of cells with epidermal growth factor ( EGF ) or after mitotic arrest and is the major K8 phosphorylated residue after incubating K8 immunoprecipitates with mitogen -activated protein or cdc2 kinases . A monoclonal antibody that specifically recognizes phosphoserine 431-K8 manifests increased reactivity with K8 and recognizes reorganized K8/18 filaments after EGF stimulation . Our results suggest that in vivo serine phosphorylation of K8 and K6e within the conserved head domain motif is likely to reflect a conserved phosphorylation site of most if not all type II keratins . Furthermore , K8 Ser-431 phosphorylation occurs after EGF stimulation and during mitotic arrest and is likely to be mediated by mitogen -activated protein and cdc2 kinases , respectively .