Fusion expression of Occludin extracellular loops and an α-helical bundle: A new research model for tight junction

: Tight junctions (TJs) are the outermost structures of intercellular junctions and are highly specialized membrane domains involved in many important cellular processes. However, most TJ proteins are four-time transmembrane proteins and are difficult to express in their correct soluble form, which limits their functional study and therapeutic application. Human occludin (OCLN) is a major component of TJs and an essential co-receptor for hepatitis C virus (HCV) cell entry. To explore expression strategy for recombinant TJ proteins possessing integrated and functional extracellular loops, OCLN was here used as a model molecule, and several prokaryotic fusion constructs were designed by docking OCLN extracellular loops (ECLs) to HIV-1 gp41 NHR and CHR six-helical bundle (6HV1); then their biophysical features and anti-HCV activity were evaluated. The proteins were successfully expressed and purified in E. coli, and the double-loop constructs (D1ECL1S+D2ECL2 as a representative) were found to have more potent HCV neutralizing activity than single-loop constructs at non-cytotoxic concentrations. Circular dichroism studies indicate that D1ECL1S+D2ECL2 adopt stable α-helical folds consistent with design. Thermal denaturation assay indicated that D1ECL1S+D2ECL2 is highly stable at 80°C (melting temperature, Tm, of 89.08 ± 2.0°C) and comparable in stability to the 6HV1 scaffold. Moreover, the time-of-addition experiment revealed that D1ECL1S+D2ECL2 predominantly functioned during the early stages of HCV entry. Taken together, these findings provide a novel strategy for recombinant TJ protein expression in vitro, which may shed light on functional and structural studies for TJs and may provide a new avenue to drug development.