U6 snRNP-binding protein that functions as a recycling factor of the splicing machinery. Promotes the initial reassembly of U4 and U6 snRNPs following their ejection from the spliceosome during its maturation. Also binds U6atac snRNPs and may function as a recycling factor for U4atac/U6atac spliceosomal snRNP, an initial step in the assembly of U12-type spliceosomal complex. The U12-type spliceosomal complex plays a role in the splicing of introns with non-canonical splice sites. May also function as a substrate-targeting factor for deubiquitinases like USP4 and USP15. Recruits USP4 to ubiquitinated PRPF3 within the U4/U5/U6 tri-snRNP complex, promoting PRPF3 deubiquitination and thereby regulating the spliceosome U4/U5/U6 tri-snRNP spliceosomal complex disassembly. May also recruit the deubiquitinase USP15 to histone H2B and mediate histone deubiquitination, thereby regulating gene expression and/or DNA repair (By similarity). May play a role in hematopoiesis probably through transcription regulation of specific genes including MYC (PubMed:21447833). Component of the 7SK snRNP complex at least composed of P-TEFb (composed of CDK9 and CCNT1/cyclin-T1), HEXIM1, HEXIM2, BCDIN3, SART3 proteins and 7SK and U6 snRNAs. Interacts with AGO1 and AGO2. Interacts with PRPF3 and USP4; the interaction with PRPF3 is direct and recruits USP4 to its substrate PRPF3. Interacts with USP15; the interaction is direct. Ubiquitously expressed, with low level of expression in liver, heart and skeletal (PubMed:10761712). Also detected in hematopoietic cells (at protein level) (PubMed:21447833). Expressed from early prenatal stages, as early as 7 dpc and increased thereafter. Up-regulated in proliferating hematopoietic cells. Knockout of Sart3 is embryonic lethal. Extended N-terminus.