Tetramers of alpha- and
beta-spectrin heterodimers, linked by intermediary
proteins to transmembrane
proteins, stabilize the red blood cell cytoskeleton. Deficiencies of either alpha- or
beta-spectrin can result in severe
hereditary spherocytosis (HS) or
hereditary elliptocytosis (HE) in mice and humans. Four mouse mutations, sph, sph(Dem), sph(2BC), and sph(J), affect the erythroid
alpha-spectrin gene, Spna1, on chromosome 1 and cause severe HS and HE. Here we describe the molecular alterations in
alpha-spectrin and their consequences in sph(2BC)/sph(2BC) and sph(J)/sph(J) erythrocytes. A splicing mutation, sph(2BC) initiates the skipping of exon 41 and premature
protein termination before the site required for dimerization of
alpha-spectrin with
beta-spectrin. A
nonsense mutation in exon 52, sph(J) eliminates the COOH-terminal 13
amino acids. Both defects result in instability of the red cell membrane and loss of membrane surface area. In sph(2BC)/sph(2BC), barely perceptible levels of
messenger RNA and consequent decreased synthesis of
alpha-spectrin protein are primarily responsible for the resultant
hemolysis. By contrast, sph(J)/sph(J) mice synthesize the truncated
alpha-spectrin in which the 13-terminal
amino acids are deleted at higher levels than normal, but they cannot retain this
mutant protein in the cytoskeleton. The sph(J) deletion is near the 4.1/actin-binding region at the junctional complex providing new evidence that this 13-amino
acid segment at the COOH-terminus of
alpha-spectrin is crucial to the stability of the junctional complex.