CUS2, A YEAST HOMOLOG OF HUMAN TAT-SF1, RESCUES FUNCTION OF MISFOLDEDU2 THROUGH AN UNUSUAL RNA RECOGNITION MOTIF

A screen for suppressors of a U2 snRNA mutation identified CUS2, an at ypical member of the RNA recognition motif (RRM) family of RNA binding proteins. CUS2 protein is associated with U2 RNA in splicing extracts and interacts with PRP11, a subunit of the conserved splicing factor SF3a. Absence of CUS2 renders certain U2 RNA folding mutants lethal, a rguing that a normal activity of CUS2 is to help refold U2 into a stru cture favorable for its binding to SF3b and SF3a prior to spliceosome assembly. Both CUS2 function in vivo and the in vitro RNA binding acti vity of CUS2 are disrupted by mutation of the first RRM, suggesting th at rescue of misfolded U2 involves the direct binding of CUS2. Human T at-SF1, reported to stimulate Tat-specific, transactivating region dep endent human immunodeficiency virus transcription in vitro, is structu rally similar to CUS2. Anti-Tat-SF1 antibodies coimmunoprecipitate SF3 a66 (SAP62), the human homolog of PRP11, suggesting that Tat-SF1 has a parallel function in splicing in human cells.