The molecular chaperones HSP28, GRP78, endoplasmin, and calnexin exhibit strikingly different levels in quiescent keratinocytes as compared to their proliferating normal and transformed counterparts: cDNA cloning and expression of calnexin.

We have identified nine molecular chaperones in human keratinocytes by one or a combination of three methods: (i) reaction with antibodies raised against the purified proteins, (ii) microsequencing of two-dimensional (2-D) gel purified proteins, or (iii), by cloning of the cDNA and expression of its encoded protein in transformed human amnion cells using the vaccinia virus expression system. The expression levels of each of the molecular chaperones were analyzed in quiescent, normal proliferating, and simian virus SV40 transformed K14 keratinocytes by cutting the corresponding protein spots from dried 2-D gels and counting the radioactivity by liquid scintillation. The most striking observation was the strong up-regulation (936%) of the small heat shock protein HSP28 in the quiescent keratinocytes, a fact that is in line with recent data indicating that the murine homologue (HSP25) may act as a growth inhibitor. Several chaperones that localize to the endoplasmic reticulum and that are involved in the secretory pathway (GRP78, GRP78v, endoplasmin, and calnexin) were expressed at approximately similar levels in normal proliferating and K14 keratinocytes but were down-regulated by 50% or more in the quiescent cells, implying that these cells may possess an impaired ability to secrete certain proteins. Both GRP78 and endoplasmin genes have similar sequences in the promoter regions, suggesting that they may be partly co-regulated at the transcriptional level (McCauliffe et al., J. Biol. Chem. 1992, 267, 2557-2562).(ABSTRACT TRUNCATED AT 250 WORDS)