The prototypical 4.1R-10-kDa domain and the 4.1G-10-kDa paralog mediate fodrin-actin complex formation

A complex family of 4.1R isoforms has been identified in non-erythroid tiss ues. In this study we characterized the exonic composition of brain 4.1R-10 -kDa or spectrin/actin binding (SAB) domain and identified the minimal sequ ences required to stimulate fodrin/F-actin association. Adult rat brain exp resses predominantly 4.1R mRNAs that carry an extended SAB, consisting of t he alternative exons 14/15/16 and part of the constitutive exon 17. Exon 16 along with sequences carried by exon 17 is necessary and sufficient to ind uce formation of fodrin-actin-4.1R ternary complexes. The ability of the re spective SAB domains of 4.1 homologs to sediment fodrin/actin was also inve stigated. 4.1G-SAB stimulates association of fodrin/actin, although with an similar to2-fold reduced efficiency compared with 4.1R-10-kDa, whereas 4.1 N and 4.1B do not. Sequencing of the corresponding domains revealed that 4. 1G-SAB carries a cassette that shares significant homology with 4.1R exon 1 6, whereas the respective sequence is divergent in 4.1N and absent from bra in 4.1B. An similar to 150-kDa 4.1R and an similar to 160-kDa 4.1G isoforms are present in PC12 lysates that occur in vivo in a supramolecular complex with fodrin and F-actin. Moreover, proteins 4.1R and 4.1G are distributed underneath the plasma membrane in PC12 cells. Collectively, these observati ons suggest that brain 4.1R and 4.1G may modulate the membrane mechanical p roperties of neuronal cells by promoting fodrin/actin association.