Intermediates in the conversion of prekeratin into keratin molecules in human epidermis

In order to elucidate the relationship between prekeratin and keratin, we performed pulse-chase experiments using [35S]methionine (35S-Met) in vitro. Of 6 prekeratin molecules (49, 52, 55, 62, 69 and 71 kDa) that incorporated 35S-Met, the 55-kDa prekeratin incorporated the most 35S-Met. In 3 molecules (52, 55 and 62 kDa) incorporation was decreased at 30 min after being chased; however, incorporation of only two molecules (55 and 62 kDa) of the 6 prekeratins was increased at 60 min. From these results and our previous data, we conclude that the initial stage of processing is as follows: 3 prekeratin molecules (52, 55 and 62 kDa) are first cleaved in the N-terminal region, then two prekeratin molecules (55 and 69 kDa) are processed to intermediates (52 and 62 kDa) by some proteolytic enzyme(s).