Isolation, characterization, and UV-stimulated expression of two families of genes encoding polypeptides of related structure in human epidermal keratinocytes.

By screening of a cDNA library made on mRNA isolated from UV-irradiated human epidermal keratinocytes for sequences whose relative concentration increases in the cytoplasm after irradiation, we have isolated 40 cDNA clones (T. Kartasova, B. J. C. Cornelissen, P. Belt, and P. van de Putte, Nucleic Acids Res. 15:5945-5962, 1987). Here we describe two distinct groups of cDNA clones which do not cross-hybridize to each other but nevertheless encode proteins of very similar primary structure. These polypeptides are small (8 to 10 kilodaltons) and exceptionally rich in proline, cysteine, and glutamine and have similar repeating elements not found elsewhere. The new proteins were designated sprI and sprII (small, proline rich). The presence of prolines and cysteines suggests that they may be either structural proteins with a strong secondary structure or metal-binding proteins such as metallothioneins. Southern blot and sequence analyses of the cDNAs indicate that at least the sprII group of clones represents a family of related genes. The nucleotide sequence of both groups seems to be conserved upon evolution. The level of mRNAs corresponding to the two groups of cDNAs is increased in the cytoplasm of human epidermal keratinocytes after both UV irradiation and treatment with 4-nitroquinoline 1-oxide or 12-O-tetradecanoylphorbol 13-acetate.