CDNA CLONING AND CHARACTERIZATION OF THE HUMAN U3 SMALL NUCLEOLAR RIBONUCLEOPROTEIN COMPLEX-ASSOCIATED 55-KILODALTON PROTEIN

The eukaryotic nucleolus contains a large number of small RNA molecule s (snoRNAs) which, in the form of small nucleolar ribonucleoprotein co mplexes (snoRNPs), are involved in the processing and modification of pre-rRNA. The most abundant and one of the best-conserved snoRNAs is t he U3 RNA. So far, only one human U3 snoRNA-associated protein, fibril larin, has been characterized. Previously, the U3 snoRNPwas purified f rom CHO cells, and three proteins of 15, 50, and 55 kDa were found to copurify with the U3 snoRNA (B. Lubben, C. Marshallsay, N. Rottmann, a nd R. Luhrmann, Nucleic Acids Res. 21:5377-5385, 1993). Here we report the cDNA cloning and characterization of the human U3 snoRNP-associat ed 55-kDa protein, The isolated cDNA codes for a novel nucleolar prote in which is specifically associated with the U3 snoRNA. This protein, referred to as hU3-55k, is the first characterized U3 snoRNP-specific protein from humans. hU3-55k is a new member of the family of WD-40 re peat proteins and is conserved throughout evolution. It appears that t he C-terminal end of hU3-55k is required for nucleolar localization an d U3 snoRNA binding.