Replication stress response protein that accumulates at DNA damage sites and promotes replication fork progression and integrity. Recruited to stalled replication forks via interaction with the RPA complex and directly stimulates ATR kinase activity independently of TOPBP1. Probably only regulates a subset of ATR targets. Interacts (via RBM1 motif) with RPA1. Interacts (via RBM2 motif) with RPA2. Interacts (via the ATR-activation domain motif) with ATR. Localizes at sites of DNA damage following replication stress. Recruited to stalled replication forks via interaction with RPA1 and RPA2 subunits of the RPA complex. The RBM1 (RPA1-binding, also named RPA70N-binding) motif mediates interaction with RPA1. The RBM2 (RPA2-binding, also named RPA32C-binding) motif mediates interaction with RPA2. The ATR-activation domain (AAD) motif is required to bind and activate ATR. Phosphorylated by ATR. It is uncertain whether Met-1 or Met-10 is the initiator.