Functional and physical interaction between the yeast splicing factors Slu7 and Prp18.

We show that the requirement for Prp18 during the second step of actin pre-mRNA splicing in vitro is dictated by the distance between the branch point and the 3'splice site. Prp18 is dispensable for splicing of precursor RNAs in which the interval between the branch point and 3'splice site is <12 nt. This resembles the requirement for another second step factor, Slu7. Excess Slu7 protein can bypass the need for Prp18 in vitro , suggesting that Slu7 and Prp18 function in a concerted manner. Physical interaction between Slu7 and Prp18 was demonstrated by using the two-hybrid assay. Deletion mutants of SLU7 were tested for their ability to support growth of a slu7 null strain. Removal of 199 amino acids from the N-terminus of the 382 amino acid Slu7 protein did not affect cell viability at 25 degrees C. A more extensive N-terminal deletion of 221 amino acids was lethal, as was a C-terminal deletion of 47 amino acids. Deleted versions of Slu7 were also tested for interaction with Prp18 in the two-hybrid system. We define a segment of Slu7 from residue 200 to 224 that is necessary for interaction with Prp18.