Spectrin is a chimeric E2/E3 ubiquitin conjugating/ligating enzyme

Abstract

Spectrin, long known as a major structural component of the membrane skeleton of erythrocytes and other cell types, has been demonstrated to have chimeric E2/E3 ubiquitin conjugating and ligating activity. Utilizing recombinant alpha spectrin peptides, covering the alpha 20/21 repeats to the C-terminus, we have demonstrated that the critical E2/E3 sites are cysteines 2071 and 2100 in human RBC spectrin. Either site can transfer ubiquitin to target sites within the same peptide segment. On the RBC membrane spectrin ubiquitinates ankyrin, band 3, protein 4.1, protein 4.2, gi 13278939 (unknown protein) and itself. Ubiquitination of alpha spectrin repeats 20/21 decreases the affinity of the spectrin-4.1-actin and spectrin-adducin-actin ternary complexes, but has no effect on heterodimer formation. Sickle cell RBCs have diminished spectrin E2/E3 activity, decreased ubiquitination of membrane proteins including spectrin, and ternary complexes that dissociate poorly. This is thought to be a major contributor to formation of the irreversibly sickled cell. This work was supported by NIH Sickle Cell Center Grant HL 070588 Project 1 (SRG PI).