Schmidt A, Langbein L, Rode M, Prätzel S, Zimbelmann R, Franke W W
Division of Cell Biology, German Cancer Research Center, Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany.
Cell Tissue Res. 1997 Dec;290(3):481-99. doi: 10.1007/s004410050956.
The cytokeratin-binding, basic 80.5 kDa polypeptide plakophilin 1 ("band 6 protein" of bovine muzzle desmosome fractions) has originally been described as a single molecular species, localized to desmosomal plaques of certain cell types, mostly stratified squamous epithelia and complex epithelia. We now report that this protein exists in at least two different isoforms: 726 amino acids (aa), plakophilin 1a; and 747 aa, plakophilin 1b. This reflects the splicing of the 21 aa-encoding exon 7 of the human plakophilin-1 gene and that each mRNA splice form can occur in two polyadenylation forms of 2.7 kb and 5.3 kb. Antibodies recognizing either isoform and/or others that are specific for the exon-encoded sequence of form 1b have allowed, in combination with immunolocalization protocols minimizing losses of diffusible proteins, the detection of both isoforms in the nucleoplasm of diverse kinds of cultured cells and tissues, including desmosome-forming cells as well as cells that never form desmosomes. The protein has also been identified in manually isolated nuclei (germinal vesicles) of Xenopus laevis oocytes. Plakophilin 1a accumulates in nuclei as shown by suitable immunolocalization protocols and upon overexpression following transfection with cDNAs, but is also located in desmosomes of stratified and complex epithelia. By contrast, isoform 1b has been found exclusively in nuclei, even in cells connected by desmosomes immunostained with plakophilin 1a-reactive antibodies. We conclude that plakophilins 1a and 1b are constitutive nuclear proteins encoded by the same gene, which is not expressed in relation to epithelial differentiation pathways, whereas the additional appearance of plakophilin 1a in desmosomal plaques of stratified and complex epithelia is regulated by an as yet unknown mechanism of differentiation-dependent topogenic recruitment. Possible functions of plakophilins are discussed in relation to recent reports of the involvement of other members of the armadillo/plakoglobin multigene family of proteins in cell surface-gene regulation signalling pathways.
细胞角蛋白结合性的、80.5 kDa的碱性多肽桥粒斑蛋白1（牛口鼻部桥粒组分的“6带蛋白”）最初被描述为单一分子类型，定位于某些细胞类型的桥粒斑，主要是复层鳞状上皮和复层上皮。我们现在报告该蛋白至少以两种不同的异构体形式存在：726个氨基酸（aa）的桥粒斑蛋白1a；以及747个aa 的桥粒斑蛋白1b。这反映了人类桥粒斑蛋白-1基因中编码21个氨基酸的外显子7的剪接情况，并且每种mRNA剪接形式都可以以2.7 kb和5.3 kb的两种聚腺苷酸化形式出现。识别任一异构体和/或对1b型外显子编码序列具有特异性的其他抗体，与将可扩散蛋白损失降至最低的免疫定位方案相结合，使得在包括形成桥粒的细胞以及从不形成桥粒的细胞在内的各种培养细胞和组织的核质中检测到了这两种异构体。该蛋白也已在非洲爪蟾卵母细胞的手工分离核（生发泡）中被鉴定出来。通过合适的免疫定位方案以及用cDNA转染后过表达显示，桥粒斑蛋白1a在细胞核中积累，但也位于复层和复层上皮的桥粒中。相比之下，1b型异构体仅在细胞核中被发现，即使在被桥粒斑蛋白1a反应性抗体免疫染色的由桥粒连接的细胞中也是如此。我们得出结论，桥粒斑蛋白1a和1b是由同一基因编码的组成型核蛋白，该基因的表达与上皮分化途径无关，而桥粒斑蛋白1a在复层和复层上皮的桥粒斑中的额外出现是由一种尚未知的依赖分化的拓扑性募集机制调控的。关于犰狳/桥粒珠蛋白多基因家族蛋白的其他成员参与细胞表面 - 基因调控信号通路的近期报道，讨论了桥粒斑蛋白可能的功能。
