Membrane proteins synthesized by human reticulocytes and their precursors.

Membrane protein synthesis in human immature erythroid cells was studied by incubating the cells with 35S-methionine in vitro. The radioactive precursor amino acid was incorporated into membrane protein in a linear fashion for approximately 60 min, after which there was only a slight increase in incorporation. Intracellular protein synthesis, in contrast, was linear for periods up to 2 h. Analysis of isolated membranes by polyacrylamide gel electrophoresis in sodium dodecyl sulphate showed that peripheral blood reticulocytes synthesized two proteins in the 4.5 region (MW=50-60000 D) and a third protein coinciding with band 6 (glyceraldehyde-3-phosphate dehydrogenase). Separation of reticulocytes into different age groups on stractan II gradients showed more immature reticulocytes synthesize a wider range of membrane proteins, extending from bands 4.1 to 8. When nucleated red cells were present in the incubations, synthesis of band 3 was also observed. Earlier erythroid precursor present in erythropoietic BFU-E cultures synthesized spectrin in addition to the other membrane proteins. The data indicate that human red cell membrane protein synthesis follows a programmed pattern and that as the erythroid elements mature they lose the capacity to synthesize certain membrane proteins.