The annexins are an evolutionarily conserved family of phospholipid-binding proteins of largely unknown function. We observed that the AnxA2

Many alleles of human disease genes have mutations within splicing consensus sequences that activate cryptic splice sites. In Caenorhabditis elegans, the unc-73(e936) allele has a G-to-U mutation at the first base of the intron downstream of exon 15, which results in an uncoordinated phenotype. This mutation triggers cryptic splicing at the À1 and 123 positions and retains some residual splicing at the mutated wild-type (wt) position. We previously demonstrated that a mutation in sup-39, a U1 snRNA gene, suppresses e936 by increasing splicing at the wt splice site. We report here the results of a suppressor screen in which we identify three proteins that function in cryptic splice site choice. Loss-of-function mutations in the nonessential splicing factor smu-2 suppress e936 uncoordination through changes in splicing. SMU-2 binds SMU-1, and smu-1(RNAi) also leads to suppression of e936. A dominant mutation in the conserved C-terminal domain of the C. elegans homolog of the human tri-snRNP 27K protein, which we have named SNRP-27, suppresses e936 uncoordination through changes in splicing. We propose that SMU-2, SMU-1, and SNRP-27 contribute to the fidelity of splice site choice after the initial identification of 59 splice sites by U1 snRNP. P RE-mRNA splicing takes place in a large ribonucleoprotein complex called the spliceosome (Burge et al. 1999). Components of this splicing machinery assemble at conserved signal sequences within the premRNA. The 59 splice site consensus sequence M À3 A À2 G À1 j G 11 U 12 R 13 A 14 G 15 U 16 and the 39 splice site consensus sequence Y À3 A À2 G À1 j R 11 (M is either A or C; R is a purine, and Y is a pyrimidine) define the limits of the intron. Base-pairing interactions between the 59 end of the U1 snRNA and the 59 splice site consensus sequence occur early in spliceosome assembly. It is the nearly invariable GU dinucleotide at the first two positions of the 59 end of the intron that defines the beginning of the intron. The 59 consensus sequence is essential but insufficient for splice site selection, as 59 splice sites with weaker consensus