UTILIZATION OF AN 86-BP EXON GENERATES A NOVEL ADDUCIN ISOFORM (BETA-4) LACKING THE MARCKS HOMOLOGY DOMAIN

A novel isoform of P-adducin has been amplified and characterized from a human bone marrow cDNA library (GenBank #U43959). This isoform aris es from the insertion of an 86 bp alternatively spliced and previously unrecognized exon (now termed exon 15) within codon 581 of the human red blood cell beta-adducin sequence. This results in an insertion of 28 novel amino acids. The remainder of the red cell beta-adducin mRNA is then translated in a different reading frame, adding an additional 35 novel amino acids prior to the stop codon. This new isoform, thus, replaces beta 1-adducin sequence after residue 580 with a total of 63 new amino acids. Sequences from genomic clones of the human beta-adduc in gene show that this alternate exon is flanked by splice consensus s equences and is appropriately located in the genomic map between exons encoding up-stream and down-stream sequences, thus defining a new exo n. The COOH-terminus of this new isoform, which we designate beta 4, l acks a 22 amino acid lysine-rich sequence common to both the human red cell alpha- and beta-adducin subunits and homologous to a highly cons erved region in MARCKS, a filamentous actin-cross linking protein regu lated by protein kinase C and calcium/calmodulin. beta 4-adducin prese rves a previously identified calmodulin binding domain. PCR analysis i ndicates that this new beta-adducin isoform is expressed in fetal brai n and liver, bone marrow, and NT-2 (neuroepithelial) cells, but is not detected in several other tissues. We anticipate that this new beta 4 isoform of beta-adducin will display unique and tissue-specific funct ional properties. (C) 1998 Elsevier Science B.V.