Conformational and biochemical characterization of a rat epididymis-specific lipocalin 12 expressed in Escherichia coli

Lipocalin 12 (Lcn12) is a recently identified epididymis-specific protein that might play a significant physiological role in male reproduction. However, the detailed structure and function of Lcn12 remain to be determined. In the present work, we cloned, expressed, and purified the rat Lcn12 (rLcn12) protein in Escherichia coli, introduced the Cys176Ala substitution to eliminate the aggregation problem associated with the wild-type protein. Homology modeling results demonstrated that rLcn12 adopted an eight-stranded, antiparallel β-barrel conformation containing a conserved disulfide bond between Cys98 and Cys203, which was in accordance with the physicochemical properties elucidated by a combination of mass, circular dichroism, and nuclear magnetic resonance spectrometry. The purified rLcn12 protein exhibited a high binding affinity for all-trans retinoic acid in fluorescence titration experiments, implying that rLcn12 could be involved in retinoic acid transport in the epididymis.

Research Highlights
► We established a convenient and efficient method for obtaining well-structured rLcn12 proteins with a high yield.
► The purified isotopically labeled protein samples are highly stable and suitable for further structural and functional studies of rLcn12.
► Our results demonstrate that rLcn12 possesses a high affinity for retinoic acid in vitro, implying that rLcn12 could be involved in retinoic acid transport in the epididymis.