Two separable cytoplasmic pools of native ribosomal subunits in chick embryo tissue culture cells

Abstract

1. 1. The cytoplasmic ribosomal particles of chick embryo tissue culture cells can be separated into two fractions each containing native 40-S and 60-S ribosomal subunits, 80-S ribosomes and polysomes. One fraction (free fraction) contains the ribosomal particles which are soluble after cell lysis without detergent. The ribosomal particles of the other fraction (membrane-associated fraction) can be solubilized only by detergent treatment of the nuclear and membrane pellet and are thus suggested to be membrane-associated. 2. 2. The relative specific activity of the ribosomal particles in the two fractions was determined by prelabeling cells with 32P followed by a pulse label with [ 3H]uridine. The native 40-S and 60-S ribosomal subunits of the free fraction have a higher relative specific activity than the corresponding particles of the membrane-associated fraction. Within both fractions the subunits have a higher specific activity than the polysomes, and the polysomes have a higher specific activity than the 80-S ribosomes. 3. 3. The possible roles of membrane-associated native ribosomal subunits as precursors in the formation of membrane-associated polysomes are discussed.