Human Fc gamma RI (CD64) is an integral membrane glycoprotein functioning as a high-affinity receptor binding to monomeric IgG. In this study, the extracellular region of Fc gamma RI, which is the actual part that interacts with IgG, was expressed as aglycosylated recombinant human Fc gamma RI (rhFc gamma RI) in Escherichia coli. The soluble form of aglycosylated rhFc gamma RI was expressed in the periplasm of E. coli. The production of soluble aglycosylated rhFc gamma RI was increased by low induction levels. Furthermore, this production was increased by low translational efficiency, controlled by modification of the putative region between the ribosome binding site and initiation codon of rhFc gamma RI fusing signal peptide (MalE, PelB, or TorT) of the expression vector. By the optimization of induction and translational efficiency, the production of soluble aglycosylated rhFc gamma RI was up to approximately 0.8 mg/l of culture medium. Surface plasmon resonance analysis revealed that the binding affinities of aglycosylated rhFc gamma RI for human IgG1 (equilibrium dissociation constant K (D) = [1.7 +/- 0.2] x 10(-10) M) and IgG3 (K (D) = [1.1 +/- 0.2] x 10(-10) M) were similar to those of glycosylated rhFc gamma RI.