DESQUAMIN IS AN EPIDERMAL RIBONUCLEASE

Desquamin is a glycoprotein that we have isolated from the upper granu lar layer and the stratum corneum of human epidermis; it is not ordina rily expressed in submerged cultures, whose terminal differentiation s tops short of formation of these layers. The exogenous addition of des quamin to human cultured keratinocytes extended their maturation, and hematoxylin staining indicated a loss of cell nuclei. For confirmation , cultured cells were lysed in situ, and the nuclei were incubated wit h desquamin for several days, then stained with hematoxylin. Damage to the nuclei was evident: the nuclear inclusions remained intact, while the surrounding basophilic nuclear matrix was degraded. Desquamin was then tested directly for nuclease activity. Ribonuclease activity was determined by incubating desquamin with human epidermal total RNA and monitoring the dose-dependent disappearance of the 28S and 18S riboso mal RNA bands in an agarose/formaldehyde gel. On RNA-containing zymoge ls, we confirmed the RNase activity to be specific to desquamin. Using synthetic RNA homopolymers, we found the active RNase domains to be l imited to cytosine residues. On the contrary, DNA was not degraded by an analogous procedure, even after strand-separation by denaturation. (C) 1998 Wiley-Liss, Inc.