oa Actin and Keratin are Binding Partners of the 1,25D3-MARRS Receptor/PDIA3/ERp57

We have shown that the 1,25D3-MARRS receptor is necessary for the rapid, pre-genomic effects of 1,25(OH)2D3 on phosphate and/or calcium absorption in chick intestines. However, a clear understanding of the proteins involved in the signaling mechanisms by which the 1,25D3-MARRS receptor facilitates 1,25(OH)2D3-mediated phosphate or calcium uptake, as well as other cellular effects, is still under investigation. We used co-immunoprecipitation studies and mass spectroscopy to identify actin and keratin as proteins that interact with the 1,25D3-MARRS receptor. Using confocal microscopy, we visualized 1,25(OH)2D3- MARRS receptor localizations relative to actin and/or keratin distribution in chick enterocytes. Cells cultured in media containing phenol red had the 1,25D3-MARRS receptor and actin localized largely in the nucleus, which was dispersed upon addition of (OH)2 1,25(OH)2D3. In the absence of phenol red, staining was cytoplasmic. Addition of steroid caused diminished staining at 10 s and 30 s, with a return of intensity between 1 and 5 min. Nuclear staining was observed after 1 min. We found that F-actin concentrations are maximal when 1,25D3-MARRS receptor localizations within enterocytes are low suggesting that cyclical conversions of F-actin to G-actin are involved in the 1,25(OH)2D3-mediated redistribution of the 1,25D3-MARRS receptor within the cell. We also found that keratin distribution remains constant with 1,25(OH)2D3 exposure when Factin depolymerizes into G-actin, which suggests that actin and keratin work in concert to facilitate hormonemediated redistribution of the 1,25D3-MARRS receptor. We subsequently investigated whether the cyclical redistribution was related to either 1,25(OH)2D3-stimulated phosphate or calcium uptake, but no congruent pattern was found.