MAMMALIAN SPLICING FACTOR SF1 IS ENCODED BY VARIANT CDNAS AND BINDS TO RNA

Mammalian splicing factor SF1 consists of a single polypeptide of 75 k Da and is required for the formation of the first ATP-dependent splice osomal complex. Three cDNAs encoding variant forms of SF1 have been is olated and four highly related cDNAs have been found in current databa ses, Comparison of the cDNA sequences suggests that different SF1 mRNA s are generated by alternative splicing of a common pre-mRNA, In agree ment with this idea, at least three mRNAs that are differentially expr essed in different cell types have been detected by northern blot anal ysis, All SF1 cDNAs identified encode proteins with a common N-termina l half that contains two structural motifs implicated in RNA binding ( an hnRNP K homology [KH] domain and a zinc knuckle), but the proteins differ in the length of a proline-rich region and have distinct C-term ini, Three SF1 isoforms expressed in insect cells via baculovirus tran sfer vectors show comparable activities in the assembly of a presplici ng complex, Consistent with the presence of a KH domain and a zinc knu ckle, we show that SF1 binds directly to RNA, This interaction appears to be largely sequence-independent with a preference for guanosine- a nd uridine-rich sequences. The KH domain of SF1 is embedded in a 160-a mino acid sequence that is shared with human Sam68, a target of Src du ring mitosis, as well as Caenorhabditis elegans GLD-1 and mouse Qkl, b oth of which play roles during cellular differentiation. The presence of this shared region in SF1 suggests functions in addition to its rol e in pre-spliceosome assembly.