Band 3 and glycophorin A (GPA) are the 2 most abundant integral proteins in the human erythrocyte membrane. Earlier studies suggested that the 2 proteins may associate not only in the mature erythrocyte membrane, but also during their posttranslational processing and intracellular trafficking. The purpose of this study was to directly examine the GPA-band 3 interaction in vivo and determine the nature of this association during erythroid membrane biogenesis. Transgenic mice were generated expressing the human glycophorin A gene and were used to examine how the induction of human GPA expression affected the levels of murine GPA and band 3 expression in the red cell membrane. Murine GPA expression was reduced in erythrocytes expressing human GPA, whereas the level of band 3 expression remained constant, implying a tight coupling of band 3 and GPA expression in the membrane of mature red cells. In vivo GPA dimerization was not modulated solely by the GPA transmembrane motif, but the distance between this motif and the basic residues on the cytoplasmic side of the transmembrane domain may also be important. In addition, GPA monomers with vary- IntroductionThe 2 most abundant integral proteins in the human erythrocyte membrane are the sialoglycoprotein glycophorin A (GPA) and the anion exchanger band 3. GPA, present at 800 000 copies per cell, is a class I transmembrane protein assembled into dimers in the mature erythrocyte membrane. [1][2][3] Human GPA undergoes homodimerization by associations between hydrophobic membranespanning domains. 4,5 With its high sialic acid content, GPA is the main contributor to the net negative cell-surface charge and is thus critical for minimizing cell-cell interactions and preventing red cell aggregation. In contrast, erythrocyte band 3 is a multifunctional protein with its N-and C-terminal domains on the cytoplasmic face of the lipid bilayer and approximately 14 membrane spans. [6][7][8] The C-terminal 52-kd membrane domain (residues 360-919) of band 3 is responsible for anion transport across the membrane, whereas the 40-kd N-terminal cytoplasmic domain plays a crucial structural role in linking the bilayer with the spectrin-based skeletal network. There are 1 ϫ 10 6 copies of band 3 per cell, which are present as a mixture of dimers and tetramers. Approximately 40% to 60% of band 3 appears to be associated with the spectrin-based skeleton.Although evidence to support a direct interaction between GPA and band 3 has remained equivocal, intriguing data suggest that the 2 proteins may associate not only in the mature erythrocyte membrane, but also during their posttranslational processing and/or intracellular trafficking during erythropoiesis. Evidence supporting an interaction in the plasma membrane includes the finding that binding of antibody to the extracellular domain of GPA decreases both the rotation 9 and lateral mobility 10 of band 3. In addition, the expression of blood group antigen Wr b requires the surface expression of both GPA and band 3 11,12 and appears to involve ...

The growth and differentiation of hematopoietic stem cells are highly dependent on regulatory molecules produced by stromal cells of the marrow environment. Evidence has accumulated over the past years which shows that adhesive receptors on hematopoietic cells and their ligands on stromal cells and extracellular matrix play a crucial role in these interactions. Integrins of the β1 family, mostly VLA-4 and VLA-5, are the best characterized and have been identified on committed progenitor cells of the hematopoietic hierarchy as well as on more primitive stem cells defined by their long-term repopulating capacity assayed in vitro as well as in vivo. Functional assays demonstrate that most progenitor cells efficiently bind to ECM components through β1 integrins and lineage- and maturation stage-specific differences have been described. Evidence exists on the direct control of late erythroid differentiation by VLA-4, but whether or not the triggering of β1 integrins is critically required for hematopoietic stem cell functioning at more immature steps is unclear. Many other integrin and non-integrin receptors involved in adhesive interactions are expressed on hematopoietic progenitor cells and tightly regulated during differentiation but their function is still controversial. Our main purpose in this review is to describe recent advances in the knowledge of integrin expression on hematopoietic progrenitor cells in both mouse and man. The emerging importance of the synergy between integrins and cytokine signalling pathways in the regulation of hematopoietic differentiation will also be discussed.