The nucleotide and deduced amino acid sequences of the coding regions of human and rat keratinocyte transglutaminases (protein-glutamine: amine y-glutamyltransferase; EC 2.3.2.13) have been determined. These yield proteins of -90 kDa that are 92% identical, indicative of the conservation of important structural features. Alignments of amino acid sequences show substantial similarity among the keratinocyte transglutaminase, human clotting factor XIII catalytic subunit, guinea pig liver tissue transglutaminase, and the human erythrocyte band-4.2 protein. The keratinocyte enzyme is most similar to factor XIII, whereas the band-4.2 protein is most similar to the tissue transglutaminase. A salient feature of the keratinocyte transglutaminase is its 105-residue extension beyond the N terminus of the tissue transglutaminase. This extension and the unrelated activation peptide of factor XIII (a 37-residue extension) appear to be added for specialized functions after divergence of the tissue transglutaminase from their common lineage.During terminal differentiation, keratinocytes of the epidermis and other stratified squamous epithelia synthesize an envelope consisting of cross-linked protein beneath the plasma membrane (1). Localization of the envelope appears due to the presence of a membrane-bound transglutaminase (2, 3) and several of its substrate proteins at the cell periphery (4). The enzyme is anchored in the membrane by acylated fatty acid (5) and is activated by flux of Ca2+ into the cytoplasm when cellular membranes lose their integrity during the final maturation stage (6). The biochemical events resulting in mature envelopes have been difficult to follow due to the intractable nature of the highly cross-linked product. In view of the many proteins and amines in keratinocytes serving as transglutaminase substrates (4, 7), further study of the enzyme structure may help in analysis of this process. In addition to acylation, for example, phosphorylation of the membrane anchorage region has been seen, which could alter the interaction of the enzyme with potential substrate proteins (8).The blood clotting factor XIII catalytic subunit (9-11) and tissue transglutaminase (12) have recently been cloned and sequenced. These enzymes are distantly related to each other but display significant similarity in certain regions, especially around the active site. An origin of the latter region in common with thiol proteases has been proposed (9). The more recent demonstration of striking similarity between the active site and a corresponding region in the erythrocyte band-4.2 protein (13, 14), however, indicates that closer relatives of transglutaminases exist. A cDNA clone for the keratinocyte-specific enzyme of the rabbit was originally identified by using an oligonucleotide probe directed toward the active site and was partially sequenced (15). Using that clone as probe, we have now cloned and determined the complete primary structure of this third type of transglutaminase for the human and rat.O Although many type...

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A cDNA library was constructed from polyadenylated RNA present in squamous differentiated rabbit tracheal epithelial cells. Screening of the cDNA library was aimed at identifying RNAs that were abundant in squamous cells and expressed at low levels in undifferentiated cells. Two different recombinants were obtained containing inserts, 0.86 and 0.77 kilobases (kb) in size, that hybridized to mRNAs 1.0 and 1.25 kb in length. These RNAs were present at approximately 50-fold higher levels in squamous cells than in proliferative or confluent retinoic acid-treated cells. The increase in the levels of the 1.0-and 1.25-kb RNAs correlated closely with the onset of squamous difrerentiation and was not related to induction of terminal cell division. Treatment of rabbit tracheal epithelial cells with transforming growth factor IS, which induces squamous differentiation in these cells, also resulted in elevated levels of the 1.0-and 1.25-kb RNAs. The increased levels of these RNAs in squamous cells appeared to a large extent to be regulated at a posttranscriptional level. Retinoic acid not only inhibited the increase in the levels of the 1.0-and 1.25-kb RNAs but also reversed the expression of these RNAs in squiamous ceUs. These results suggest that retinoic acid affects, directly or indirectly, molecular events that induce alterations in the posttranscriptional processing of the transcripts corresponding to the 1.0-and 1.25-kb RNAs.

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