Effector Fc gamma receptors (Fc gamma Rs) are expressed on the surface of a variety of cells of hematopoietic lineage and serve as a bridge between adaptive and innate immune responses. The interaction between immune complexes, formed by IgG class antibodies that are crosslinked with antigen, and FcgRs triggers signaling cascades that result in numerous cellular responses including the activation or donwregulation of cytotoxic responses, cytokine release, and antibody synthesis. Here, the extracellular domains of the human type I transmembrane FcgRs were expressed in Escherichia coli and their interactions to subclass IgGs (IgG1, IgG2, IgG3, and IgG4) antibodies were analyzed. Expression using fully synthetic E. coli codon optimized Fc gamma R genes and optimization of sequences for N-terminal translation initiation region through mRNA secondary structure prediction enabled us to achieve high yield of purified, bacterially expressed receptors, including Fc gamma RI and Fc gamma RIIIa which have not been successfully expressed in bacteria until now. The aglycosylated Fc gamma Rs showed similar IgG subclass binding selectivity compared to the respective glycosylated FcgRs expressed in mammalian cells. Biotechnol. Bioeng. 2010; 107: 21-30. (C) 2010 Wiley Periodicals, Inc.