The nucleolar antigen Nop52, the human homologue of the yeast ribosomal RNA processing RRP1, is recruited at late stages of nucleologenesis

We report the molecular characterization of a novel nucleolar protein, Nop5 2, and its subcellular distribution during the cell cycle and nucleologenes is. This protein was originally identified with human autoantibodies which were subsequently used to clone its corresponding cDNA. Transfection experi ments in mammalian cells have confirmed that this cDNA encodes a nucleolar protein that accumulates in the nucleoli and at the periphery of the chromo somes, Nop52 is the putative human homologue of the yeast ribosomal RNA pro cessing protein RRP1 which is involved in pre-rRNA processing from 27S to 2 5S and 5.8S, In nucleoli, Nop52 is excluded from the ribosomal RNA transcri ption sites, accumulates in the granular external domain and mainly colocal izes with nucleolar proteins involved in the late processing step such as h Pop1 and protein B23, During the building process of the nucleolus at the e nd of mitosis, a sequential order was observed in the assembly of nucleolar proteins of early and late processing mainly via the prenucleolar body pat hway. The order is the following: fibrillarin, nucleolin, Nop52 together wi th protein B23 in the prenucleolar bodies, and simultaneously with hPop1, a nd finally Ki-67, The evolutionary conservation of Nop52 and the lethal eff ects observed in gene disruption experiments, predict a critical role for N op52 in the generation of 28S rRNA.