Maturation and Intranuclear Transport of Pre-Ribosomes Requires Noc Proteins

How pre-ribosomes temporally and spatially mature during intranuclear biogenesis is not known. Here, we report three nucleolar proteins, Noc1p to Noc3p, that are required for ribosome maturation and transport. They can be isolated in two distinct complexes: Noc1p/Noc2p associates with 90S and 66S pre-ribosomes and is enriched in the nucleolus, and Noc2p/Noc3p associates with 66S pre-ribosomes and is mainly nucleoplasmic. Mutation of each Noc protein impairs intranuclear transport of 60S subunits at different stages and inhibits pre-rRNA processing. Overexpression of a conserved domain common to Noc1p and Noc3p is dominant-negative for cell growth, with a defect in nuclear 60S subunit transport, but no inhibition of pre-rRNA processing. We propose that the dynamic interaction of Noc proteins is crucial for intranuclear movement of ribosomal precursor particles, and, thereby represent a prerequisite for proper maturation.