The dynamic phosphorylation of the human intermediate filament keratin 1 chain

The major sites of phosphorylation have been determined on the human keratin intermediate filament keratin 1 (type II) chain expressed in terminally differentiating epidermis. A total of nine phosphate sites were found, involving 1 threonine and 8 serine residues, and were localized to end domain sequences. The sites identified corresponded to major sites of phosphorylation as determined by direct quantitation of O-phosphoserine. Since the tissue was cultured with [32 P] orthophosphate only briefly, labeling occurred primarily by turnover, so that information on the dynamics of phosphorylation was also obtained. The degrees and specific activities (that is, turnover rates) of phosphorylation of these sites varied widely between different isoelectric variants (phosphate isomers) of keratin 1 chains and correlated with their locations on the chain: those sites on the more exposed E1 and E2 subdomains were fully phosphorylated and turning over at high rates, while a site near the end of the rod domain in a presumably more confined location was only slightly phosphorylated and turning over at low rate. The nature of the sequences around the phosphorylated residues indicates that cAMP-dependent and probably other protein kinase activities operate simultaneously in intact normal epidermal tissue. The correlation between the degrees and rates of turnover of phosphorylation with the locations on the chain may have an important bearing on the functional role of phosphorylation of the keratin intermediate filaments in this tissue.