Human epidermal proteolipids: isolation, partial characterization, and subcellular localization.

Since the first description of organic-soluble proteins (i.e., proteolipids), much attention has focused on the isolation, purification, characterization, localization, and function of these intrinsic membrane proteins in a variety of different tissues. Using a rapid purification scheme, which allowed the transfer of organic-soluble proteolipids to aqueous phases, we have isolated proteolipids from cultured human keratinocytes and human epidermis for the first time. A partial characterization of these proteolipids, including molecular-weight determination by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), amino acid composition, and an N-terminal sequencing revealed a preponderance of hydrophobic amino acids (greater than 60% overall and greater than 78% in N-terminal sequence), typical of other proteolipids. The composition of fatty acids, covalently bound to whole purified apoprotein fractions, displayed a predominance of palmitic greater than oleic greater than stearic acids. Comparison of the molecular species of proteolipids isolated from whole epidermis with those obtained from keratinocyte cultures by SDS-PAGE revealed a comparable spectrum of apoprotein species. Finally, subcellular fractionation of cultured keratinocytes, used to localize proteolipids to specific cellular compartments, suggested that one of the major apoprotein species (30 kD) is present in mitochondria, whereas the lower molecular weight species are localized in plasma membrane-enriched fractions. Although evidence is lacking for a specific function(s) of this class of molecules in the epidermis, the hypothesis that it plays a role in epidermal differentiation, for example, as constituents of calcium and/or proton pumps, is discussed.