Identification of human macrophage inflammatory proteins 1 alpha and 1 beta as a native secreted heterodimer

Chemokines are secreted proteins that function as chemoattractants for leuk ocytes. The chemokines macrophage inflammatory protein 1 alpha and 1 beta ( MIP-1 alpha and MIP-1 beta) now have been shown to be secreted from activat ed human monocytes and peripheral blood lymphocytes (PBLs) as a heterodimer . Immunoprecipitation and immunoblot analysis revealed that antibodies to e ither MIP-1 alpha or MIP-1 beta precipitated a protein complex containing b oth MIP-1 alpha and MIP-1 beta under normal conditions from culture superna tants and lysates of these cells. Mass spectrometry of the complexes, preci pitated from the culture supernatants of monocytes and PBLs, revealed the p resence of NH2-terminal truncated MIP-1 alpha (residues 5-70) together with either intact MIP-1 beta or NH2-terminal truncated MIP-1 beta (residues 3- 69), respectively. The secreted MIP-1 alpha/beta heterodimers were dissocia ted into their component monomers under acidic conditions. Exposure of mono cytes or PBLs to monensin induced the accumulation of heterodimers composed of NH2-terminal truncated MTP-1 alpha and full-length MIP-1 beta in the Go lgi complex. The mixing of recombinant chemokines in vitro demonstrated tha t heterodimerization of MIP-1 alpha and MIP-1 beta is specific and that it occurs at physiological conditions, pH 7.4, and in the range of nanomolar c oncentrations. The data presented here provide the first biochemical eviden ce for the existence of chemokine heterodimers under natural conditions. Fo rmation of heterodimers of MIP-1 alpha/beta may have an impact on intracell ular signaling events that contribute to CCR5 and possibly to other chemoki ne receptor functions.