The cytokine portion of p43 occupies a central position within the eukaryotic multisynthetase complex.

Multicellular eukaryotes contain a macromolecular assembly of nine aminoacyl-tRNA synthetase activities and three auxiliary proteins. One of these, p43, is the precursor of endothelial monocyte-activating polypeptide II (EMAP II), an inflammatory cytokine involved in apoptotic processes. As a step toward understanding this paradoxical association, the EMAP II portion of p43 has been localized within the rabbit reticulocyte multisynthetase complex. Immunoblot analysis demonstrates strong reaction of anti-EMAP II antiserum with p43, as well as cross-reactivity with isoleucyl-tRNA synthetase. Electron microscopic images of immunocomplexes show two antibody binding sites. The primary site is near the midpoint of the multisynthetase complex at the intersection of the arms with the base. This site near the lower edge of the central cleft is assigned to the C-terminal cytokine portion of p43. The secondary site of antibody binding is in the base of the particle and maps the location of isoleucyl-tRNA synthetase. These data allow refinement of the three-domain model of polypeptide distribution within the multisynthetase complex. Moreover, the central location of p43/EMAP II suggests a role for this polypeptide in optimizing normal function and in rapid disruption of essential cellular machinery when apoptosis is signaled.