Purification and physical properties of homogeneous initiation factor MP from rabbit reticulocytes.

Initiation factor MP was purified 1570-fold with 67% recovery of total activity present in 0.5 M KCl extracts of rabbit reticulocyte ribosomes. Initiation factor MP forms a ternary complex with Met-tRNAf and GTP or a binary complex with Met-tRNAf alone, the details of which are presented in the accompanying paper (Safer, B., Adams, S. L., Anderson. W. F., and Merrick, W. C. (1975) J. Biol. Chem. 250, 9076-9082). Initiation factor MP was homogeneous by the following criteria: (a) electrophoresis as a single band in gels of 5, 6, 7, 8, 9, and 10% acrylamide; (b) equilibration as a single band during isoelectric focusing; (c) sedimentation as a single symmetrical boundary during sedimentation velocity experiments; (d) linear plots of sedimentation equilibrium data; (e) symmetrical absorbance (at 280 nm) and activity profiles during DEAE-cellulose and Sephadex G-200 chromatography, and (f) symmetrical distribution of initiation factor MP during sucrose density gradient band sedimentation. The molecular weight of the initiation factor MP monomer (0.2 mg/ml) by low speed sedimentation equilibrium was 90,800. Calculations based on the Stokes radius and sedimentation velocity show the existence of relatively stable 90,000-dalton monomers or 180,000-dalton dimers at low (0.1 mg/ml) and high (9.75 mg/ml) concentrations of initiation factor MP, respectively. Electrophoresis in sodium dodecyl sulfate gels indicates that initiation factor MP monomer is composed of two noncovalently linked subunits with molecular weights of 52,000 and 34,000. Despite a relatively normal amino acid composition and an isoelectric point of 6.4, initiation factor MP behaves as a basic protein, eluting from phosphocellulose at 650 mM KCl (pH 7.9). Both ternary complex formation and methionyl-puromycin synthesis co-purify, indicating that a single protein is required for both activities.