Identification and characterization of a phospholipid scramblase encoded by planarian Dugesia japonica

Phospholipid scramblases (PLSCRs) are the conserved calcium-binding, type II transmembrane proteins synthesized in all eukaryotic organisms. In mammals, these proteins play essential roles in various physiological processes, especially in the immune responses. However, the existence of PLSCRs and their biological functions in planarian are still unknown at present. In this study, a new member of PLSCRs was identified in planarian Dugesia japonica (D. japonica), named DjPLSCR. The sequence analysis revealed that it contains an opening reading frame consisting of 726bp encoding a putative protein of 241 amino acids with a predicted molecular mass of ~28.7kDa and an isoelectric point of 6.21. Whole-mount in situ hybridization showed that mRNAs of DjPLSCR are predominantly expressed in adult and regenerative pharynx which is an important organ of immune system in planarians. Importantly, we found that the transcription level of DjPLSCR was significantly upregulated when planarians were stimulated with the pathogen-associated molecular patterns [polyinosinic-polycytidylic acid, lipopolysaccharide, peptidoglycan and β-glucan], suggesting that DjPLSCR is involved in the immune response upon pathogen invasion. Our findings provide the first experimental insights into the characteristics and potential functions of PLSCR in planarians.