Abstract
During erythroblast enucleation, nuclei surrounded by plasma membrane separate from erythroblast cytoplasm. A key aspect of this process is sorting of membrane components to plasma membranes surrounding expelled nuclei and young reticulocytes. This protein partitioning performs a crucial role in regulating the protein content of reticulocyte plasma membranes. Although it is known that cytoskeletal actin, spectrin and protein 4.1R distribute to reticulocytes, little is known about the sorting patterns of erythroblast transmembrane proteins. In hereditary spherocytosis (HS) and hereditary elliptocytosis (HE), erythrocytes contain well-described deficiencies of various transmembrane proteins, in addition to those encoded by the mutant genes. For example, elliptocytic human and murine erythrocytes resulting from mutations in the 4.1R gene lack not only protein 4.1R but also transmembrane protein glycophorin C (GPC), known to be a 4.1R binding partner with a key role in linking cytoskeleton to bilayer. Similarly, in HS resulting from mutations in the ankyrin gene, deficiencies of band 3, Rh and GPA have been documented. Various molecular mechanisms could explain deficiencies of membrane proteins in HS and HE erythrocytes including: perturbed trafficking to the erythroblast membrane; aberrant protein sorting during erythroblast enucleation; and selective loss during reticulocyte membrane remodeling. We explored whether aberrant protein sorting during enucleation might be responsible for GPC deficiency in HE. First we performed immunochemical analysis of the sorting pattern of GPC using highly purified extruded nuclei and immature reticulocytes derived from terminally differentiated murine erythroblast cultures. Proteins from equivalent numbers of expelled nuclei and reticulocytes were analyzed by Western blotting. Using antibodies specific for GPC we determined that 90% of GPC sorted to reticulocyte plasma membranes. To validate these results we used live cell, three-color immunofluorescent microscopy and analyzed enucleating erythroblasts, reticulocytes and extruded nuclei from freshly harvested murine wild type bone marrow. Independently confirming the Western blot data, we found that GPC sorted almost exclusively to reticulocytes with little or no GPC in association with nuclear plasma membrane. Strikingly, in 4.1R null erythroblasts GPC was distributed exclusively to expelled nuclei. These findings unequivocally establish that skeletal protein 4.1R is critical for normal sorting of GPC to young reticulocytes and provide clear evidence that specific skeletal protein associations can regulate protein sorting during enucleation. Moreover, our data provide a molecular explanation for the underlying basis of GPC deficiency observed in 4.1R-deficient individuals with HE. We speculate that aberrant protein sorting may be a prevalent mechanism for the deficiencies of various membrane proteins in HS and HE and that their differential loss could contribute to the variable phenotypic expression of these hemolytic disorders.
SEC18/NSF-independent, protein-sorting pathway from the yeast cortical ER to the plasma membrane
