CELL-TYPE-SPECIFIC DESMOSOMAL PLAQUE PROTEINS OF THE PLAKOGLOBIN FAMILY - PLAKOPHILIN-1 (BAND-6 PROTEIN)

Desmosomes represent a special type of the plaque-bearing adhering jun ctions, characteristic of certain pathways of cell differentiation, wh ich compositionally are not identical in the various kinds of desmonos ome-forming cells. While all desmosomes contain the cytoplasmic plaque proteins desmoplakin I and plakoglobin, they can vary in their specif ic complement of desmosomal cadherins and by the presence of additiona l plaque proteins. We have raised monoclonal antibodies recognizing on e such 'accessory' plaque protein, the cytokeratin-binding, basic prot ein plakophilin 1, originally introduced as 'band 6 protein' or 'polyp eptide D6', which is an abundant desmosomal component in certain epith elia. Using such antibodies, we have isolated cDNA clones encoding the bovine and the human protein and determined their complete amino acid sequences. The mRNAs, which on Northern blot tests appear as two band s corresponding to approximately 4 and 2.4 kb (bovine) or 5 and 2.6 kb (human), code for 727 amino acids (calculated mel. wt. 80,180; IEP 9. 25) in bovine and 726 amino acids (mel. wt. 80,496; IEP 9.34) in human plakophilin. Sequence analyses have revealed the presence of 9.2 repe ated units of the arm-motif sequence, confirming our previous conclusi on that this protein is a member of a larger family of proteins includ ing, inter alia, sever-al membrane-associated plaque proteins such as vertebrate plakoglobin and beta-catenin as well as the product of the armadillo gene of Drosophila. The plakophilin antibodies and cDNA prob es have also allowed us to examine its synthesis in various tissues an d cell cultures. While we confirm the occurrence of the protein in cyt oskeletal fractions from various stratified squamous, complex, glandul ar duct and bladder epithelia, where it can be localized to desmosomes , we have, surprisingly, also identified the protein, although at lowe r amounts, in cytoskeletal fractions from several cultured cell lines in which the protein has not been consistently localized to desmosomes by immunofluorescence microscopy. Examples include cultured cells der ived from certain simple epithelia such as the kidney-derived line MDB K and cultured calf lens cells. We have also found that, in all plakop hilin 1-positive cells examined, a pool of diffusible ('soluble') cyto plasmic plakophilin exists, including cell lines such as human mammary carcinoma MCF-7 cells in which this soluble plakophilin seems to be t he only detectable form. In addition, we have identified some soluble proteins conspicuously cross-reacting with plakophilin 1. Possible fun ctions of plakophilin and its potential value as a marker for specific states of cell differentiation are discussed, particularly with respe ct to tumor diagnosis.