The lipocalin XLCpl1 expressed in the neural plate ofXenopus laevisembryos is a secreted retinaldehyde binding protein

Open Access
Publisher Website
Google Scholar
Abstract
The cellular and structural properties and binding capabilities of a lipocalin expressed in the early neural plate ofXenopus laevisembryos and the adult choroid plexus have been investigated. It was found that this lipocalin, termed Xlcpll, binds retinal at a nanomolar concentration, retinoic acid in the micromolar range, but does not show binding to retinol. Furthermore, this protein also binds D/L thyroxine. TheXlcpl1cDNA was expressed in cell culture using the vaccinia virus expression system. In AtT20 cells, Xlcpl1 was secreted via the constitutive secretory pathway. We therefore assume that cpl1 binds retinaldehyde during the transport through the compartments of the secretory pathway that are considered to be the storage compartments of retinoids. Therefore, cpll‐expressing cells will secrete the precursors of active retinoids such as retinoic acid isomers. These retinoids may enter the cytosol by diffusion or receptor‐controlled mechanisms, as has been shown for exogenously applied retinoids. Based on these data, it is suggested that cpl1 is an integral member of the retinoid signaling pathway and, therefore, it plays a key role in pattern formation in early embryonic development.