Structure of initiation factor eIF‐3 from rat liver

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Abstract
On the basis of hydrodynamic, electron microscopic and biochemical investigations a new model of the structure of initiation factor elF-3 is proposed. From sedimentation and diffusion coefficients of 16.35 S and 2.13 .times. 10-7 cm2/s, respectively, as well as from sedimentation equilibrium measurements, a molecular mass of about 650 kDa was determined for isolated elF-3. This is in agreement with molecular mass estimations by sodium dodecyl sulphate gel electrophoresis. A partial specific volume of 0.723 cm3/g was determined by means of the amino acid composition and the specific volume increments of the amino acids. From this value and from the molecular mass, a volume of 780 nm3 was calculated for elF-3. In electron micrographs of isolated elF-3, images with triangular profiles and side lengths of 14 nm, 16 nm, and 17 nm have been observed. Taking into account the calculated volume and considering the triangular image as one face of the particle, it is suggested that elF-3 has the shape of a flat triangular prism with a height of about 7 nm and the above-mentioned side-lengths. This model is in agreement with results of electron microscopic investigations of elF-3 in native small ribosomal subunits [Lutsch, G., Benndorf, R., Westermann, P., Bommer, U.-A. and Bielka, H. (1986) Eur. J. Cell Biol. 40/2, in press]. The high frictional ratio of about 1.7 also supports elF-3 to be rather of a flat than of a globular shape.