Demonstration of the Molecular Shape of BP180, a 180-kDa Bullous Pemphigoid Antigen and Its Potential for Trimer Formation

Abstract

The 180-kDa bullous pemphigoid antigen (BP180) is a hemidesmosomal transmembrane glycoprotein comprising interrupted collagen domains in its extracellular part. BP180 is also termed type XVII collagen. But the question of whether it actually takes a collagen-like triple helical conformation in vivo has remained unanswered. Using a monoclonal antibody, we found that a subpopulation of BP180 localizes at the lateral surfaces of corneal basal cells and cultured cells, in addition to the basal surface. This subpopulation of BP180 could be solubilized by 0.5% Triton X-100 and, among examined cell lines, was found to be most abundant in BMGE+H, a bovine mammary gland epithelial cell line. The Triton-soluble fraction of BMGE+H cells was used for characterization. On sucrose gradient centrifugation, the soluble BP180 demonstrated a value of approximately 7 S, and chemical cross-linking experiments revealed a trimer form. The calculated frictional ratio, f/f0 = 2.8, suggests an asymmetric configuration. For further characterization, we purified the soluble BP180 by immunoaffinity column chromatography using an anti-BP180 monoclonal antibody. Rotary shadowing images of the purified BP180 showed a quaver-like molecule consisting of a globular head, a central rod, and a flexible tail. With regard to the primary structure and species comparisons, the central rod, 60-70 nm in length, probably corresponds to the largest collagenous region, forming a collagen-like triple helix, in human form. The globular head and the flexible tail seem to correspond to the cytoplasmic and the interrupted collagenous region, respectively, of the extracellular portions. In conclusion, the present demonstration of the entire configuration of BP180, with a collagen-like trimer in its extracellular part, suggests that BP180 is one of the major components of anchoring filaments.