The spliceosomal snRNP core complex of Trypanosoma brucei: Cloning and functional analysis reveals seven Sm protein constituents

Each of the trypanosome small nuclear ribonucleoproteins (snRNPs) U2, U4/U6 , and U5, as well as the spliced leader (SL) RNP, contains a core of common proteins, which we have previously identified. This core is unusual becaus e it is not recognized by anti-Sm Abs and it associates with an Sm-related sequence in the trypanosome small nuclear RNAs (snRNAs). Using peptide sequ ences derived from affinity-purified U2 snRNP proteins, we have cloned cDNA s for five common proteins of 8.5, 10, 12.5, 14, and 15 kDa of Trypanosoma brucei and identified them as Sm proteins SmF (8.5 kDa), -E (10 kDa), -D1 ( 12.5 kDa), -G (14 kDa), and -D2 (15 kDa), respectively. Furthermore, we fou nd the trypanosome SmB (T. brucei) and SmD3 (Trypanosoma cruzi) homologues through database searches, thus completing a set of seven canonical Sm prot eins. Sequence comparisons of the trypanosome proteins revealed several dev iations in highly conserved positions from the Sm consensus motif. We have identified a network of specific heterodimeric and -trimeric Sm protein int eractions in vitro. These results are summarized in a model of the trypanos ome Sm core, which argues for a strong conservation of the Sm particle stru cture. The conservation extends also to the functional level, because at le ast one trypanosome Sm protein, SmG, was able to specifically complement a corresponding mutation in yeast.