The spatio-temporal organization of proteins within the cytoplasm of eukaryotic cells rests in part on the assembly of stable and transient multiprotein complexes. Here we examined the assembly of the multiaminoacyl-tRNA synthetase complex (MARS) in human cells. This complex contains nine aminoacyl-tRNA synthetases and three auxiliary proteins and is a hallmark of metazoan species. Isolation of the complexes has been performed by tandem affinity purification from human cells in culture. To understand the rules of assembly of this particle, expression of the three nonsynthetase components of MARS, p18, p38, and p43, was blocked by stable small interfering RNA silencing. The lack of these components was not lethal for the cells, but cell growth was slightly reduced. The residual complexes that could form in vivo in the absence of the auxiliary proteins were isolated by tandem affinity purification. From the repertoire of the subcomplexes that could be isolated, a comprehensive map of protein-protein interactions mediating complex assembly is deduced. The data are consistent with a structural role of the three nonsynthetase components of MARS, with p38 connecting two subcomplexes that may form in the absence of p38.