Component of the spliceosome A complex. Regulates alternative splicing of a number of mRNAs. May modulate splice site pairing after recruitment of the U1 and U2 snRNPs to the 5' and 3' splice sites of the intron (By similarity). Component of the spliceosome A complex (also known as the prespliceosome). Appears to dissociate from the spliceosome upon formation of the spliceosome B complex (also known as the precatalytic spliceosome), in which the heterotrimeric U4/U6.U5 snRNPs are bound (By similarity). Belongs to the RBM5/RBM10 family.