Molecular characterization of the body site-specific human epidermal cytokeratin 9: cDNA cloning, amino acid sequence, and tissue specificity of gene expression.

Differentiation of human plantar and palmar epidermis is characterized by the suprabasal synthesis of a major special intermediate-sized filament (IF) protein, the type I (acidic) cytokeratin 9 (CK 9). Using partial amino acid (aa) sequence information obtained by direct Edman sequencing of peptides resulting from proteolytic digestion of purified CK 9, we synthesized several redundant primers by 'back-translation'. Amplification by polymerase chain reaction (PCR) of cDNAs obtained by reverse transcription of mRNAs from human foot sole epidermis, including 5'-primer extension, resulted in multiple overlapping cDNA clones, from which the complete cDNA (2353 bp) could be constructed. This cDNA encoded the CK 9 polypeptide with a calculated molecular weight of 61,987 and an isoelectric point at about pH 5.0. The aa sequence deduced from cDNA was verified in several parts by comparison with the peptide sequences and showed the typical structure of type I CKs, with a head (153 aa), and alpha-helical coiled-coil-forming rod (306 aa), and a tail (163 aa) domain. The protein displayed the highest homology to human CK 10, not only in the highly conserved rod domain but also in large parts of the head and the tail domains. On the other hand, the aa sequence revealed some remarkable differences from CK 10 and other CKs, even in the most conserved segments of the rod domain. The nuclease digestion pattern seen on Southern blot analysis of human genomic DNA indicated the existence of a unique CK 9 gene. Using CK 9-specific riboprobes for hybridization on Northern blots of RNAs from various epithelia, a mRNA of about 2.4 kb in length could be identified only in foot sole epidermis, and a weaker cross-hybridization signal was seen in RNA from bovine heel pad epidermis at about 2.0 kb. A large number of tissues and cell cultures were examined by PCR of mRNA-derived cDNAs, using CK 9-specific primers. But even with this very sensitive signal amplification, only palmar/plantar epidermis was found positive. By in situ hybridization and immunolocalization we further showed that CK 9 is only expressed in the suprabasal cell layers of this special epidermal tissue. We discuss the molecular properties of CK 9 and its cell type- and body site-specific expression in relation to the special differentiation of palmar/plantar epidermis and to diseases specific for this body site.