The two forms of the β-subunit of initiation factor-2 from reticulocyte lysates arise from proteolytic degradation

Dholakia and Wahba (J. Biol. Chem. (1987) 262, 10164-10170) have reported that preparations of purified initiation factor-2 (eIF-2) from rabbit reticulocytes contain two forms of the β-subunit. These forms differ in their apparent molecular weights as judged by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), and are accordingly termed β_H (heavy, the slower-migrating species, apparent M_r = 54300) and β_L (light, the faster-migrating species, apparent M_r = 53 100). We confirm that two forms of eIF-2β are present in such preparations, but present evidence that the β_L is generated from β_H during the isolation procedure. Crude reticulocyte lysates contain only the β_H species as judged from immunoblotting of reticulocyte proteins resolved by SDS-PAGE using an antiserum against eIF-2β. The β_L species appears after the ammonium sulphate fractionation step used early in the purification procedure, but is not apparent if a cocktail of proteinase inhibitors is included in the buffers used during the purification, indicating that it is a proteolytic degradation product generated during the isolation procedure. Cleveland mapping failed to reveal any differences between the two species. Both the β_H and the β_L forms are phosphorylated by casein kinase-2, and, as judged by one- and two-dimensional peptide mapping, at identical sites in each species. Since casein kinase-2 phosphorylates serine-2 in eIF-2β, the β_L form must still contain the N-terminal region and is presumably produced by limited proteolysis at the carboxyl terminus of the β-subunit.