Cinétique de marquage des protéines nucléolaires: Mise en évidence d'une fraction rapidement marquée

Abstract 1. 1. The possibilities of a transfer towards the nucleolus of proteins neo-synthesized elsewhere has been examined. The difference in specific activity evolution of nucleolar proteins and of the total cellular proteins has been determined through two types of experiments: During continuous labelling, either in vitro or in vivo; During a chase after a short pulse with 3 H- l -leucine (5 min). In the first case, the specific activity ratio between nucleolar proteins and the total cellular proteins increases constantly during the first minutes of labelling and reaches a plateau after about 30 min. In the second case, the specific activity of nucleolar proteins is increased three-fold during the first 20 min of the chase, while the specific activity of the total cellular proteins remains stable. These results confirm that the major part of labelled proteins, located in the nucleolus some 30 min after labelling has started, were indeed synthesized outside the nucleolus. 2. 2. On the other hand, complete lysis of isolated nucleoli, obtained by heparin, facilitated the liberation of various RNP types containing ribosomal RNAs precursors. Kinetics of protein labelling of the various components resulting from the nucleolar lysis, has shown, under the same conditions, the early labelling (as compared with that of the various types of nucleolar RNP) of the lightest nucleolar fraction which is very rich in proteins. The possible location, in this fraction, of proteins newly arrived in the nucleolus before having taken part in the manufacturing of preribosomal RNP, is discussed.