Maturation of a 100 kDa protein associated with preribosomes in Chinese hamster ovary cells

A 100 kDa nucleolar protein which is transitorily associated with preribosomes in the nucleoli of Chinese hamster ovary cells has been found to be specifically cleaved by a thiol protease. During an 'in vitro' incubation of nucleoli, the 100 kDa protein is processed into eight different proteins which are detected by immunoreaction with a serum raised against the 100 kDa protein. Qualitative and quantitative variations in the maturation products of the 100 kDa protein are obtained by 'in vitro' incubation of the 60S and 80S preribosomes. The 100 kDa protein has been purified to homogeneity with the protease activity still associated. The properties of the enzyme are described and its role in the maturation of preribosomes is discussed.