Elliptocytosis in patients with C-terminal domain mutations of protein 4.1correlates with encoded messenger RNA levels rather than with alterations in primary protein structure

Early biochemical studies defined 4 functional domains of the erythroid pro tein 4.1 (4.1R). From amino-terminal to carboxy-terminal, these are 30 kd, 16 kd, 10 kd, and 22/24 kd in size. Although the functional properties of b oth the 30-kd and the 10-kd domain have been demonstrated in red cells, no functional activities have been assigned to either the 16-kd or the 22/24-k d domain in these cells. We here describe new mutations in the sequence enc oding the C-terminal 22/24-kd domain that are associated with hereditary el liptocytosis. An unusually mild phenotype observed in heterozygous and homo zygous members of 1 family suggested heterogeneity in the pattern of expres sion of 4.1R deficiency. Using a variety of protein and messenger RNA (mRNA ) quantification strategies, we showed that, regardless of the alteration i n the C-terminal primary sequence, when the protein is produced, it assembl es at the cell membrane. In addition, we found that alterations in red cell morphologic features and membrane function correlate with the amount of me mbrane-associated protein-and there fore with the amount of mRNA accumulate d-rather than with the primary structure of the variant proteins. These dat a suggest that an intact sequence at exons 19 through 21 encoding part of t he C-terminal 22/24-kd region is not required for proper protein 4.1R assem bly in mature red cells. (C) 2000 by The American Society of Hematology.