Keratin intracellular concentration revisited: Implications for keratin function in surface epithelia

In 1978 Sun and Green carried out a densitometry-based analysis of polyacrylamide gelelectrophoresed samples and reported that keratin proteins account for 25~35% of total cellular proteins in human epidermal keratinocytes serially passaged in primary culture. This represents an astounding figure with very significant implications for the structural support role of keratin intermediate filaments (IFs) in epidermis, and its regulation by posttranslational modifications and/or interaction with associated proteins. We report here on an effort to investigate this issue further, using a distinct method of keratin quantitation in sorted but otherwise native cell populations, and taking advantage of the significant progress made in our understanding of the structure of keratin filaments and differential regulation of keratin expression in epidermis.