A role for phosphorylation in the dynamics of keratin intermediate filaments

Keratins undergo highly dynamic events in the epithelial cells that express them. These dynamic changes have been associated with important cell proce sses. We have studied the possible role of keratin phosphorylation-dephosph orylation processes in the control of these dynamic events. Drugs that affe ct the protein phosphorylation metabolism (activators or inhibitors of prot ein kinases or protein phosphatases) have been used in two different dynami c experimental systems. First, the behaviour of keratins after the formatio n of cell heterokaryons, and second, the assembly of a newly synthesised ke ratin after transfection into the pre-existing keratin cytoskeleton. The ma in difference between these two systems stems on the alteration of the amou nt of keratin polypeptides present in the cells, since in heterokaryons thi s amount was unaltered whilst in transfection experiments there is an incre ase due to the presence of the transfected protein. We observed in both sys tems that the inhibition of protein kinases led to a delayed dynamic behavi our of the keratin polypeptides. On the contrary; the inhibition of protein phosphatases by okadaic acid or the activation of protein kinases by phorb ol esters promoted a substantial increase in the kinetics of these processe s. Biochemical studies demonstrate that this behavioural changes can be cor related with changes in the phosphorylation state of the keratin polypeptid es. As a whole, present results indicate that the highly dynamic properties of the keratin polypeptides can be modulated by phosphorylation.