The Clf1p splicing factor promotes spliceosome assembly through N-terminal tetratricopeptide repeat contacts

Spliceosome assembly follows a well conserved pathway of subunit addition that includes both small nuclear ribonucleoprotein (snRNP) particles and non-snRNP splicing factors. Clf1p is an unusual splicing factor composed almost entirely of direct repeats of the tetratricopeptide repeat (TPR) protein-binding motif. Here we show that the Clf1p protein resides in at least two multisubunit protein complexes, a small nuclear RNA-free structure similar to what was reported as the Prp19p complex (nineteen complex; NTC) and an RNP structure that contains the U2, U5, and U6 small nuclear RNAs. Thirty Ccf (Clf1p complex factor) proteins have been identified by mass spectroscopy or immune detection as known or suspected components of the yeast spliceosome. Deletion of TPR1 or TPR2 from an epitope-tagged Clf1p protein (i.e. CIf1Delta2-TAP) destabilizes Clf1p complexes assembled in vivo, causing the release of the Cef1p and Prp19p NTC factors and decreased association of the Rse1p, Snu114p, and Hsh155p snRNP proteins. In vitro, temperature inactivation of Clf1Delta2p impairs the prespliceosome to spliceosome transition and prevents Prp19p recruitment to the splicing complex. These and related data support the view that the poly-TPR Clf1p splicing factor promotes the functional integration of the U4/U6.U5 tri-snRNP particle into the U1-U2-dependent prespliceosome.