A NEW 440-KD ISOFORM IS THE MAJOR ANKYRIN IN NEONATAL RAT-BRAIN

This report describes initial characterization of a 440-kD isoform of brain ankyrin (ankyrin(B)) representing an alternatively spliced mRNA product of the gene encoding the major isoform of ankyrin in adult human brain (Otto, E., M. Kunimoto, T. McLaughlin, V. Bennett, J. Cell Biology. 114:241-253). Northern and immunoblot analyses indicate that 440-kD ankyrin(B) includes the spectrin and membrane-binding domains as well as a regulatory domain of the major 220-kD isoform. 440-kD ankyrin(B) contains, in addition, a sequence of a predicted size of 220 kD which is inserted between the regulatory domain and spectrin/membrane-binding domains. 440-kD ankyrin(B) has properties expected of a peripherally associated membrane-skeletal protein: it is exclusively present in the particulate fraction of brain homogenates, is extracted with NaOH, and remains associated with Triton-X-100-resistant structures. Expression of 440-kD ankyrin(B) in rat brain began at birth before other ankyrins could be detected, peaked 10 d after birth, and then decreased progressively to 30% of the maximum in adults. Expression of the 220-kD ankyrin(B) and ankyrin(R) (erythroid ankyrin) began approximately 10 d after the 440-kD isoform, increased rapidly between 10 and 15 d after birth, and finally achieved their maximal levels in adults. 440-kD ankyrin(B) is present in approximately equivalent amounts in all regions of neonatal brain while in adult brain it is present in highest levels in cerebellum and lowest in brain stem. 440-kD ankyrin(B) was localized by immunofluorescence in regions of neonatal and adult brain containing primarily dendrites and unmyelinated axons. 440-kD ankyrin(B) thus may play a specialized role in neuronal processes.