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Molecular and Cellular Biology, August 1999, p. 5441-5452, Vol. 19, No. 8
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Imp3p and Imp4p, Two Specific Components of the U3 Small Nucleolar Ribonucleoprotein That Are Essential for Pre-18S rRNA Processing

Sarah J. Lee1 and Susan J. Baserga1,2,*

Department of Therapeutic Radiology and Genetics1 and Yale Cancer Center,2 Yale School of Medicine, New Haven, Connecticut 06520-8040

Received 16 March 1999/Returned for modification 20 April 1999/Accepted 5 May 1999

The function of the U3 small nucleolar ribonucleoprotein (snoRNP) is central to the events surrounding pre-rRNA processing, as evidenced by the severe defects in cleavage of pre-18S rRNA precursors observed upon depletion of the U3 RNA and its unique protein components. Although the precise function of each component remains unclear, since U3 snoRNA levels remain unchanged upon genetic depletion of these proteins, it is likely that the proteins themselves have significant roles in the cleavage reactions. Here we report the identification of two previously undescribed protein components of the U3 snoRNP, representing the first snoRNP components identified by using the two-hybrid methodology. By screening for proteins that physically associate with the U3 snoRNP-specific protein, Mpp10p, we have identified Imp3p (22 kDa) and Imp4p (34 kDa) (named for interacting with Mpp10p). The genes encoding both proteins are essential in yeast. Genetic depletion reveals that both proteins are critical for U3 snoRNP function in pre-18S rRNA processing at the A0, A1, and A2 sites in the pre-rRNA. Both Imp proteins associate with Mpp10p in vivo, and both are complexed only with the U3 snoRNA. Conservation of RNA binding domains between Imp3p and the S4 family of ribosomal proteins suggests that it might associate with RNA directly. However, as with other U3 snoRNP-specific proteins, neither Imp3p nor Imp4p is required for maintenance of U3 snoRNA integrity. Imp3p and Imp4p are therefore novel protein components specific to the U3 snoRNP with critical roles in pre-rRNA cleavage events.

* Corresponding author. Mailing address: Dept. of Therapeutic Radiology and Genetics, Yale School of Medicine, P.O. Box 208040, New Haven, CT 06520-8040. Phone: (203) 785-4618. Fax: (203) 785-6309. E-mail: susan.baserga{at}yale.edu.

Molecular and Cellular Biology, August 1999, p. 5441-5452, Vol. 19, No. 8
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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