Required for DNA replication. Functions as part of the GINS complex which plays an essential role in the initiation of DNA replication by binding to DNA replication origins and facilitating the assembly of the DNA replication machinery. Required for the chromatin binding of CDC45. Component of the GINS complex which is a heterotetramer of SLD5, PSF1, PSF2 and PSF3. Interacts with PSF2. Associates with autonomously replicating sequence (ARS) regions in S phase. This association requires SLD3 and DPB11. Present with 1431 molecules/cell in log phase SD medium. Belongs to the GINS1/PSF1 family.