A New Spectrin, βIV, Has a Major Truncated Isoform That Associates with Promyelocytic Leukemia Protein Nuclear Bodies and the Nuclear Matrix*

We isolated cDNAs that encode a 77-kDa peptide similar to repeats 10–16 of β-spectrins. Its gene localizes to human chromosome 19q13.13-q13.2 and mouse chromosome 7, at 7.5 centimorgans. A 289-kDa isoform, similar to full-length β-spectrins, was partially assembled from sequences in the human genomic DNA data base and completely cloned and sequenced. RNA transcripts are seen predominantly in the brain, and Western analysis shows a major peptide that migrates as a 72-kDa band. This new gene, spectrin βIV, thus encodes a full-length minor isoform (SpβIVΣ1) and a truncated major isoform (SpβIVΣ5). Immunostaining of cells shows a micropunctate pattern in the cytoplasm and nucleus. In mesenchymal stem cells, the staining concentrates at nuclear dots that stain positively for the promyelocytic leukemia protein (PML). Expression of SpβIVΣ5 fused to green fluorescence protein in cells produces nuclear dots that include all PML bodies, which double in number in transfected cells. Deletion analysis shows that partial repeats 10 and 16 of SpβIVΣ5 are necessary for nuclear dot formation. Immunostaining of whole-mount nuclear matrices reveals diffuse positivity with accentuation at PML bodies. Spectrin βIV is the first β-spectrin associated with a subnuclear structure and may be part of a nuclear scaffold to which gene regulatory machinery binds.