Subcellular distribution of a thiol-proteinase inhibitor protein was determined in the epidermis of the newborn rat by light and electron microscopy. This protein was highly soluble in basal cells and concentrated on ribosomes in the perinuclear region. Solubility in Tris buffer decreased in granular and cornified cells in which the protein appeared on polysomes which were attached on other cellular structures such as dense homogenous deposits and tonofilaments. The protein also appeared to be deposited on the plasma membrane and became insoluble in Tris buffer at 37° C, but solubilized in 1 M phosphate buffer. Location of the protein around keratohyalin granules or by the plasma membrane suggested that the inhibitor protein bound to cysteinerich protein of the epidermis with or without forming a thiol-proteinase inhibitor complex. The thiol-proteinase inhibitor protein seems to contribute to epidermal cell differentiation at multiple points through changes in its solubility and subcellular localization from basal cells to cornified cells
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