Assembly of stratum corneum basic protein and keratin filaments in macrofibrils

Abstract

THE stratum corneum is the most superficial aspect of mammalian epidermis and consists of several layers of fully differentiated, dead cells filled with an insoluble protein complex called keratin. Early ultrastructural studies of the stratum corneum cells revealed a ‘keratin pattern’ of light-staining filaments 70–80 Å in diameter embedded in a dark-staining osmiophilic amorphous matrix¹. The filaments are synthesised in the inner living cell layers and are variously termed tonofilaments, α-keratin or, more accurately, keratin filaments. The subunits of the filaments of both the living cells and stratum corneum have been extracted with dissociating reagents from bovine^2–4, rat^5–6, mouse⁷, and human⁸ epidermis. In appropriate conditions, these subunits will polymerise in vitro into native-type keratin filaments that are uniformally ^70–80 Å in diameter^7,9. In contrast, the amorphous matrix has not been positively identified at the biochemical level, although it was recently suggested that a stratum corneum basic protein (SCBP) rich in histidine has properties consistent with a possible function as an interfilamentous matrix substance¹⁰. We show here that highly ordered structures reminiscent of the keratin pattern in the intact stratum corneum form when the SCBP and epidermal keratin filaments are combined in vitro.