Functional Cus1p is found with Hsh155p in a multiprotein splicing factor associated with U2 snRNA

To explore the dynamics of snRNP structure and function, we have studied Cu s1p, identified as a suppressor of U2 snRNA mutations in budding yeast, Cus 1p is homologous to human SAP145, a protein present in the 17S form of the human U2 snRNP, Here, we define the Cus1p amino acids required for function in yeast. The segment of Cus1p required for binding to Hsh49p, a homolog o f human SAP49, is contained within an essential region of Cus1p, Antibodies against Cus1p coimmunoprecipitate U2 snRNA, as well as Hsh155p, a protein homologous to human SAP155, Biochemical fractionation of splicing extracts and reconstitution of heat-inactivated splicing extracts from strains carry ing a temperature-sensitive allele of CUS1 indicate that Cus1p and Hsh155p reside in a functional, high-salt-stable complex that is salt-dissociable f rom U2 snRNA, We propose that Cus1p, Hsh49p, and Hsh155p exist in a stable protein complex which can exchange with a core U2 snRNP and which is necess ary for U2 snRNP function in prespliceosome assembly. The Cus1p complex sha res functional as well as structural similarities with human SF3b.