Domain Requirements and Genetic Interactions of the Mud1 Subunit of the Saccharomyces cerevisiae U1 snRNP

Mud1 is an inessential 298-amino acid protein subunit of the Saccharomyces cerevisiae U1 snRNP. Mud1 consists of N-terminal and C-terminal RRM domains (RRM1 and RRM2) separated by a linker domain. Synthetic lethal interactions of mud1 with deletions of inessential spliceosome components Nam8, Mud2, and Msl1, or missense mutations in the branchpoint-binding protein Msl5 enabled us to dissect genetically the domain requirements for Mud1 function. We find that the biological activities of Mud1 can be complemented by co-expressing separately the RRM1 (aa 1-127) and linker-RRM2 (aa 128-298) modules. Whereas RRM1 and RRM2 (aa 197-298) per se are inactive in all tests of functional complementation, the linker-RRM2 by itself partially complements a subset of synthetic lethal mud1 interactions. Linker segment aa 155 to 196 contains a nuclear localization signal rich in basic amino acids that is necessary for RRM2 activity in mud1 complementation. Alanine scanning mutagenesis indicates that none of the individual RRM1 amino acid contacts to U1 snRNA in the cryo-EM model of the yeast U1 snRNP is necessary for mud1 complementation activity.

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