Maturation and intranuclear transport of pre-ribosomes requires Noc proteins

How pre-ribosomes temporally and spatially mature during intranuclear bioge nesis is not known. Here, we report three nucleolar proteins, Noc1p to Noc3 p, that are required for ribosome maturation and transport. They can be iso lated in two distinct complexes: Noc1p/Noc2p associates with 90S and 66S pr e-ribosomes and is enriched in the nucleolus, and Noc2p/Noc3p associates wi th 66S pre-ribosomes and is mainly nucleoplasmic. Mutation of each Noc prot ein impairs intranuclear transport of 60S subunits at different stages and inhibits pre-rRNA processing. Overexpression of a conserved domain common t o Noc1p and Noc3p is dominant-negative for cell growth, with a defect int n uclear 60S subunit transport, but no inhibition of pre-RNA processing. We p ropose that the dynamic interaction of Noc proteins is crucial for intranuc lear movement of ribosomal precursor particles, and, thereby represent a pr erequisite for proper maturation.