Involved in the early steps of selenocysteine biosynthesis and tRNA(Sec) charging to the later steps resulting in the cotranslational incorporation of selenocysteine into selenoproteins. Stabilizes the SECISBP2, EEFSEC and tRNA(Sec) complex. May be involved in the methylation of tRNA(Sec). Enhances efficiency of selenoproteins synthesis (By similarity). Component of the tRNA(Sec) complex composed at least of EEFSEC, SECISBP2, SEPHS1, SEPSECS, TRNAU1AP and tRNA(Sec). Associates with mRNP and/or polysomes (By similarity). Found in a complex with tRNA(Sec). Interacts with SEPSECS. Abundant in the nucleus. Ubiquitous. Belongs to the RRM TRSPAP family.