Novel Alternative Splicings of BPAG1 (Bullous Pemphigoid Antigen 1) Including the Domain Structure Closely Related to MACF (Microtubule Actin Cross-linking Factor)

Abstract

BPAG1 (bullous pemphigoid antigen 1) was originally identified as a 230-kDa hemidesmosomal protein and belongs to the plakin family, because it consists of a plakin domain, a coiled-coil rod domain and a COOHterminal intermediate filament binding domain. To date, alternatively spliced products of BPAG1, BPAG1e, and BPAG1n are known. BPAG1e is expressed in epithelial tissues and localized to hemidesmosomes, on the other hand, BPAG1n is expressed in neural tissues and muscles and has an actin binding domain at the NH 2 -terminal of BPAG1e. BPAG1 is also known as a gene responsible for Dystonia musculorum (dt) neurodegeneration syndrome of the mouse. Another plakin family protein MACF (microtubule actin cross-linking factor) has also an actin binding domain and the plakin domain at the NH 2 -terminal. However, in contrast to its high homology with BPAG1 at the NH 2 -terminal, the COOHterminal structure of MACF, including a microtubule binding domain, resembles dystrophin rather than plakins. Here, we investigated RNAs and proteins expressed from the BPAG1 locus and suggest novel alternative splicing variants, which include one consisting of the COOH-terminal domain structure homologous to MACF. The results indicate that BPAG1 has three kinds of cytoskeletal binding domains and seems to play an important role in linking the different types of cytoskeletons. BPAG11 and BPAG2 are the autoantigens of bullous pemphigoid (BP), an autoimmune subepidermal skin blistering disease (1). Both are components of hemidesmosomes, and BPAG1 is a 230-kDa cytoplasmic protein and anchors keratin-containing intermediate filaments (IFs) to the inner plaque of hemidesmosomes (1-3). BPAG2 is a 180-kDa transmembrane protein with interrupted collagen domains in its extracellular part (1-3). BPAG1 belongs to the plakin family, including plectin, desmoplakin, envoplakin, and periplakin, which associate with IFs in various tissues (4, 5). Plakins have a common structure consisting of an amino (NH 2 )-terminal plakin domain, a central coiled-coil domain, and a carboxyl (COOH)-terminal domain containing the IF binding domain (4, 5). In addition, BPAG1 and plectin give rise to alternative splicing variants, which have an actin binding domain (ABD) at the NH 2 terminus (6 -11). BPAG1 with ABD expressed in nervous tissues and muscles are distinguished as BPAG1n from BPAG1e that has no ABD and localizes to the hemidesmosome in the epithelial tissue (11-13). Consistent with the expression and localization, BPAG1-deficient mice show skin blistering and neurodegeneration (12). Also, the mouse mutant, Dystonia musculorum (dt), which shows neurodegeneration but normal skin, is known to result from loss of BPAG1n but normal expression of BPAG1e (12).Mammalian protein MACF (microtubule actin cross-linking factor) and Drosophila protein Kakapo are classified as members of the plakin family (13-15). Human MACF was also reported as ABP620 or trabeculin-␣ (16, 17). They are a novel type of plakin proteins, because their COOH-terminal structures containing micr...

Collagen XVII and BPAG1 expression in the retina: Evidence for an anchoring complex in the central nervous system

“…The other high-molecular-weight protein (above 500 kDa) may represent either the novel BPAG1c isoform (see Fig. 1) or other known large isoforms, BPAG1a and BPAG1b (Leung et al, 2001;Okumura et al, 2002). In contrast, although Mü ller cells express collagen XVII, they apparently do not synthesize BPAG1 (data not shown).…”

IRBIS: a systematic search for conserved complementarity

“…A number of alternatively spliced isoforms have been reported for this gene (Okumura et al 2002), including isoforms that encode plectin type of repeats (PTRs) and spectrin type repeats (STRs). In particular, exons 47-52 that encode a group of PTRs (Punta et al 2012) are either spliced out or included as a cluster, with the exception of exon 51, which can be skipped independently (Fig.…”

The `Spectraplakins': cytoskeletal giants with characteristics of both spectrin and plakin families

“…Analysis of such large RNAs (from 8 to 27 kb) is by necessity achieved by piecing together short cDNAs, but whether the assembled sequence reflects the most abundant mRNA is less clear. Important progress has recently been made in defining the major isoforms by northern analysis (Leung et al, 2001b;Okumura et al, 2002). Examination of the gene structure reveals potential splice donor and acceptor sites that allow one to construct exceptionally large proteins, but at present some of these remain hypothetical.…”