Catalyzes the attachment of threonine to tRNA(Thr) in a two-step reaction: threonine is first activated by ATP to form Thr-AMP and then transferred to the acceptor end of tRNA(Thr). Also edits incorrectly charged tRNA(Thr) via its editing domain, at the post-transfer stage. ATP + L-threonine + tRNA(Thr) = AMP + diphosphate + H(+) + L-threonyl-tRNA(Thr) May be a component of the multisynthetase complex (MSC), a large multi-subunit complex which contains at least eight different aminoacyl-tRNA synthetases plus three auxillary subunits AIMP1, AIMP2 and EEF1E1. Interacts with the MSC components EPRS1, AIMP1, AIMP2 and KARS1. Primarily cytoplasmic. Also detected at lower levels in the nucleus. Belongs to the class-II aminoacyl-tRNA synthetase family.