Molecular and functional characterization of protein 4.1B, a novel member of the protein 4.1 family with high level, focal expression in brain

Brain-enriched isoforms of skeletal proteins in the spectrin and ankyrin ge ne families have been described. Here we characterize protein 4.1B, a novel homolog of erythrocyte protein 4.1R that is encoded by a distinct gene. In situ hybridization revealed high level, focal expression of 4.1B mRNA in s elect neuronal populations within the mouse brain, including Purkinje cells of the cerebellum, pyramidal cells in hippocampal regions CA1-3, thalamic nuclei, and olfactory bulb. Expression was also detected in adrenal gland, kidney, testis, and heart. 4.1B protein exhibits high homology to the membr ane binding, spectrin-actin binding, and C-terminal domains of 4.1R, includ ing motifs for interaction with NuMA and FKBP13, cDNA characterization and Western blot analysis revealed multiple spliceoforms of protein 4.1B, with functionally relevant heterogeneity in the spectrin-actin and NuMA binding domains. Regulated alternative splicing events led to expression of unique 4.1B isoforms in brain and muscle; only the latter possessed a functional s pectrin-actin binding domain. By immunofluorescence, 4.1B was localized spe cifically at the plasma membrane in regions of cell-cell contact. Together these results indicate that 4.1B transcription is selectively regulated amo ng neuronal populations and that alternative splicing regulates expression of 4.1B isoforms possessing critical functional domains typical of other pr otein 4.1 family members.