Stratum corneum basic protein: an interfilamentous matrix protein of epidermal keratin

The stratum corneum basic protein (SCBP) aggregates specifically with keratin filaments, forming macrofibrils in which the filaments are highly aligned. The appearance of the macrofibrils formed in vitro is similar to the keratin pattern in the intact stratum corneum, suggesting that SCBP is an interfilamentous matrix substance of epidermal keratin (Dale et all., Nature, 276: 729, 1978, see Ref. 13). Electron microscope and biochemical analyses were performed to determine the optimum molar ratio of SCBP and keratin filaments (average subunit molecular weight, 62,000). The optimum mixtures, in which all of the proteins were aggregated into macrofibrils, contained 1-2 moles of SCBP per 3 moles filament subunits. At ratios below this value, free filaments were observed by electron microscopy; above this value, excess SCBP was observed in the supernate by immunologic assay. The precursor of SCBP has been purified after extraction from keratohyalin granules in 1 M potassium phosphate. The precursor is immunologically similar to SCBP. Their amino acid compositions are essentially identical. However, in contrast to SCBP, the precursor is an insoluble, neutral protein which contains approximately 10 moles of phosphate per mole protein. This phosphate is covalently bound to the precursor and is not due to the presence of nucleic acid. During the conversion of a granular cell to a cornified cell, the precursor, present in keratohyalin granules, must be converted by proteolysis and/or dephosphorylation to active SCBP which aggregates with keratin filaments and forms the complex of filaments embedded in a matrix.