J Biol Chem, Vol. 274, Issue 43, 30393-30401, October 22, 1999
An RNA Binding Motif in the Cbp2 Protein Required for
Protein-stimulated RNA Catalysis
Hymavathi K.
Tirupati,
Lynn C.
Shaw, and
Alfred S.
Lewin
From the Department of Molecular Genetics and Microbiology,
University of Florida College of Medicine, Gainesville, Florida
32605
The fifth and terminal intron of yeast cytochrome
b pre-mRNA (a group I intron) requires a protein
encoded by the nuclear gene CBP2 for splicing. Because
catalysis is intrinsic to the RNA, the protein is believed to promote
formation of secondary and tertiary structure of the RNA, resulting in
a catalytically competent intron. In vitro, this
mitochondrial intron can be made to self-splice or undergo
protein-facilitated splicing by varying the Mg2+ and
monovalent salt concentrations. This two-component system, therefore,
provides a good model for understanding the role of proteins in RNA
folding. A UV cross-linking experiment was initiated to identify RNA
binding sites on Cbp2 and gain insights into Cbp2-intron interactions.
A 12-amino acid region containing a presumptive contact site near the
amino terminus was targeted for mutagenesis, and mutant proteins were
characterized for RNA binding and stimulation of splicing. Mutations in
this region resulted in partial or complete loss of function,
demonstrating the importance of this determinant for stimulation of RNA
splicing. Several of the mutations that severely reduced splicing did
not significantly shift the overall binding isotherm of Cbp2 for the
precursor RNA, suggesting that contacts critical for activity are not
necessarily reflected in the dissociation constant. This analysis has
identified a unique RNA binding motif of alternating basic and aromatic
residues that is essential for protein facilitated splicing.
Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.