Bpag1 localization to actin filaments and to the nucleus is regulated by its N-terminus

Plakins are a family of giant cytoskeleton binding proteins. One member of this group is bullous pemphigoid antigen 1 (Bpag1)/dystonin, which has neuronal and muscle isoforms that consist of actin-binding and microtubule-binding domains at either end separated by a plakin domain and several spectrin repeats. The better-characterized epithelial isoform has only the plakin domain in common with the neuronal and muscle isoforms. Here, we have analyzed the localization of muscle/neuronal (Bpag1a/b) isoforms and the epithelial (Bpag1e) isoform within C2C12 myoblast cells. Although an antibody specific to Bpag1a/b isoform 2 detected protein co-aligning actin stress fibers, this same antibody and two Bpag1e antibodies predominantly detected protein in the nuclei. A Bpag1a/b isoform 2 N-terminal fusion protein containing the plakin domain also localized to actin stress fibers and to nuclei. Within the plakin domain, we characterized a functional nuclear localization signal, which was responsible for localization of the fusion protein to the nucleus. Bpag1a/b isoform 1 N-terminal fusion proteins differed in their interaction with the actin cytoskeleton and with their ability to localize to the nucleus, suggesting that Bpag1 isoforms with different N-termini have differing roles. These results show the importance of N-terminal domains in dictating the localization and function of Bpag1 isoforms. We provide the first indication that Bpag1 is not strictly a cytoplasmic/membrane protein but that it can also localize to the nucleus.