Spectrin's E2/E3 ubiquitin conjugating/ligating activity is diminished in sickle cells

Abstract

Erythrocyte spectrin contains E2/E3 ubiquitin conjugating/ligating activity in its alpha subunit. Ankyrin is a target of spectrin's E2/E3 ubiquitin conjugating/ligating activity in vitro and in vivo. We compare the ubiquitination levels of ankyrin mediated by control and sickle cell spectrin using a biotinylated ubiquitin cell-free assay. Sickle cell spectrin has diminished ability to transfer ubiquitin from an intermediate spectrin-ubiquitin thioester adduct (alpha' spectrin) to ankyrin, which may be due to glutathiolation of spectrin's E2 and/or E3 active site cysteines. There is also a diminished ability of the sickle cell ankyrin to serve as target of spectrin's E2/E3 activity, probably due to oxidative damage to ankyrin. A direct correlation exists between the alpha'/alpha spectrin ratio and spectrin's ability to ubiquitinate ankyrin. There is also an inverse correlation between severity of the disease and the alpha'/alpha spectrin ratio in SS erythrocytes. These results suggest that reduced spectrin E2/E3 activity is an important determinant of sickle cell severity.