Mechanisms of apical protein sorting in polarized thyroid epithelial cells

The process leading to thyroid hormone synthesis is vectorial and depends u pon the polarized organization of the thyrocytes into the follicular unit. Thyrocyte membrane proteins are delivered to two distinct domains of the pl asma membrane using apical (AP) and basolateral (BL) sorting signals. A rec ent hypothesis for AP sorting proposes that apically destined proteins clus ter with glycosphingolipids (GSLs) and cholesterol, into microdomains (or r afts) of the Golgi membrane from which AP vesicles originate. In MDCK cells the human neurotrophin receptor, p75(hNTR), is delivered to the AP surface through a sorting signal, rich in O-glycosylated sugars, identified in its ectodomain. We have investigated whether this signal is functional in the thyroid-derived FRT cell line and whether p75(hNTR) clusters into lipid raf ts to be sorted to the AP membrane. We found that p75(hNTR) is apically del ivered via a direct pathway and does not associate with rafts during its tr ansport to the surface of FRT cells. Therefore, although the same signal co uld be recognized by different cell types thyroid cells may possess a tissu e-specific sorting machinery. (C) Societe francaise de biochimie et biologi e moleculaire / Elsevier, Paris.