Identification, characterization and crystal structure analysis of the human spliceosomal U5 snRNP-specific 15 kD protein

The U5 small ribonucleoprotein particle (snRNP) contains various proteins i nvolved in catalytic activities mediating conformational rearrangements of the spliceosome. We have isolated and characterized the evolutionarily high ly conserved human U5 snRNP-specific protein U5-15kD. The crystal structure of U5-15kD determined at 1.4 Angstrom resolution revealed a thioredoxin-li ke fold and represents the first structure of a U5 snRNP-specific protein k nown so far. With respect to human thioredoxin the U5-15kD protein contains 37 additional residues causing structural changes which most likely form p utative binding sites for other spliceosomal proteins or RNA. Moreover, a n ovel intramolecular disulfide bond replaces the canonical one found in the thioredoxin family. Even though U5-15kD appears to lack protein disulfide i somerase activity, it is strictly required for pre-mRNA splicing in vivo as we demonstrate by genetic depletion of its ortholog in Saccharomyces cerev isiae. Our data suggest that the previously reported involvement of its Sch izosaccharomyces pombe ortholog Dim1p in cell cycle regulation is a consequ ence of its essential role in pre-mRNA splicing. (C) 1999 Academic Press.