Abstract
Spectrin, a heterodimer of alpha and beta subunits, is an essential component of the red blood cell membrane skeleton. The jaundiced (ja/ja) mutation causes a severe hemolytic anemia in mice and is mapped to the erythroid beta-spectrin locus (Spnb-1) on chromosome 12. As a prerequisite for determining the molecular defect of the jaundiced mutation, we have cloned and sequenced the complete murine reticulocyte cDNA for normal Spnb-1. Two unique transcripts that differ in the placement of polyA tails are represented in the clones isolated. Amino acid sequence comparison between erythroid and murine brain spectrin (Spnb-2, chromosome 11) shows 67% identity throughout repeats 16 and 17 and complete divergence in domain III, which is associated with the alpha/beta subunit dimerization and phosphorylation. We examined the tissue distribution of normal and mutant erythroid beta-spectrin transcripts using domain-specific probes. Transcripts are detected in normal spleen tissue and reticulocytes (8 and 9.6 kb), brain tissue (10 and 11 kb), skeletal muscle tissue, and cardiac muscle tissue (11, 10.3, 7.2, and 4.0 kb). Extensive variability in mRNA processing is shown with region-specific probes. Steady state levels of the mutant transcripts are reduced when hybridized with a probe to repeats 2 through 6 with the exception of the 7.2-kb transcript that is unique to heart and skeletal muscle tissues, and is present at normal and elevated levels, respectively, in ja/ja mice. These results provide evidence for more diverse tissue-specific products of the Spnb-1 gene than were previously suspected.
