Catalyzes the specific attachment of an amino acid to its cognate tRNA in a 2 step reaction: the amino acid (AA) is first activated by ATP to form AA-AMP and then transferred to the acceptor end of the tRNA (PubMed:11706011). When secreted, acts as a signaling molecule that induces immune response through the activation of monocyte/macrophages. Catalyzes the synthesis of the signaling molecule diadenosine tetraphosphate (Ap4A), and thereby mediates disruption of the complex between HINT1 and MITF and the concomitant activation of MITF transcriptional activity (By similarity). ATP + L-lysine + tRNA(Lys) = AMP + diphosphate + L-lysyl-tRNA(Lys) Homodimer. Part of the multisynthetase complex (MSC), a multisubunit complex that groups tRNA ligases for Arg (RARS), Asp (DARS), Gln (QARS), Ile (IARS), Leu (LARS), Lys (KARS), Met (MARS) the bifunctional ligase for Glu and Pro (EPRS) and the auxiliary subunits AIMP1/p43, AIMP2/p38 and EEF1E1/p18. Interacts with AIMP2 (via N-terminus) and MITF. Interacts with TARSL2. Secretion is induced by TNF-alpha. Cytosolic in quiescent mast cells. Translocates into the nucleus in response to mast cell activation by immunoglobulin E. The N-terminal domain (1-65) is a functional tRNA-binding domain and is involved in the interaction with DARS, but has a repulsive role in the binding to EEF1A1. A central domain (208-259) is involved in homodimerization. The C-terminal domain (452-597) is not required for interaction with AIMP2 (By similarity). Phosphorylated on a serine residue after mast cell stimulation with immunoglobulin E (IgE). It is likely that the same gene provides both this cytoplasmic isoform and an additional mitochondrial isoform. Belongs to the class-II aminoacyl-tRNA synthetase family.