[Functional characteristics of two membrane-bound polysome fractions from chick embryos]

Among various membrane-bound polyribosomes from chicken embryos the polyribosomes loosely bound with membranes proved to be highly active in synthesis of total proteins as well as of collagens in vitro. These data suggest that polyribosomes loosely bound with membranes were not an impurity of free polyribosomes in the total preparation of the membrane-bound polyribosomes. These polyribosomes constituted a definite class of polyribosomes active in the synthesis of secreted proteins (i.e. of collagen). In polyribosome fractions identified by their size (monosomes, light and heavy polyribosomes) all the three fractions of loosely-bound polyribosomes as well as light and heavy fractions of tightly-bound polyribosomes were active in synthesis of total proteins. Differences between tightly-and loosely-bound polyribosomes were noted also in studies of cell-free synthesis of collagen proteins. Heavy fractions of tightly-bound polyribosomes were the most active in synthesis of these proteins, light fractions were less active and monosomes did not exhibit any collagen-synthesizing activity. Only monosomes of loosely-bound polyribosomes were responsible for the synthesis of collagen. Polyribosomes loosely-bound with membranes appear to participate at early steps of membrane-polyribosome complex formation.