Tail anchored (TA) proteins, which are important for numerous cellular processes, are defined by a single transmembrane domain (TMD) near the C-terminus. The membrane insertion of TA proteins is mediated by the highly conserved ATPase Get3. Here we report the crystal structures of Get3 in ADP-bound and nucleotide-free forms at 3.0 A and 2.8 A resolutions, respectively. Get3 consists of a nucleotide binding domain and a helical domain. Both structures exhibit a Zn(2+)-mediated homodimer in a head-to-head orientation, representing an open dimer conformation. Our cross-link experiments indicated the closed dimer-stimulating ATP hydrolysis, which might be coupled with TA-protein release. Further, our coexpression-based binding assays using a model TA protein Sec22p revealed the direct interaction between the helical domain of Get3 and the Sec22p TMD. This interaction is independent of ATP and dimer formation. Finally, we propose a structural mechanism that links ATP hydrolysis with the TA-protein insertion mediated by the conserved DTAPTGH motif.