Synthesis of keratin proteins during maturation of cultured human keratinocytes

We have examined the synthesis of keratin and total protein during cellular maturation in cultured oral and epidermal keratinocytes. Keratinocytes were separated on the basis of maturational age on gradients of Ficoll 400. Analysis of selected gradient fractions revealed that during maturation in culture the amount of protein/cell increases from 100 to 900 pg and the apparent rate of protein synthesis increases about two-fold. However, the proportion of protein that is keratin remains constant throughout this process. The major keratin proteins (58K, 56K, and 46K) are synthesized at the same relative rates throughout maturation in vitro. Therefore, during terminal differentiation in culture, the keratinocyte forms an expanding cell in which the rate of protein synthesis increases while the relative proportion of keratin proteins and nonkeratin proteins remain the same.