Acts as a specific nuclear import carrier for HSP70 proteins following heat-shock stress: acts by mediating the nucleoporin-dependent translocation of ATP-bound HSP70 proteins into the nucleus. HSP70 proteins import is required to protect cells from heat shock damages. Does not translocate ADP-bound HSP70 proteins into the nucleus (By similarity). Forms an asymmetric homodimer; required for binding and nuclear import of HSP70 proteins. Interacts with ATP-bound HSP70 proteins. Interacts with NUP62 and NUP153 (via F-X-F-G repeats). Interacts with HSPA8. Belongs to the OPI10 family.