Detergent insoluble microdomains ave not involved in transcytosis of polymeric Ig receptor in FRT and MDCK cells

In polarized epithelial cells, sorting of proteins and lipids to the apical or basolateral domain of the plasma membrane can occur via direct or indir ect (transcytotic) pathways from the trans Golgi network (TGN). The 'rafts' hypothesis postulates that the key event for direct apical sorting of some transmembrane proteins and the majority of GPI-anchored proteins depends o n their association with glycosphingolipid and cholesterol enriched microdo mains (rafts). However, the mechanism of indirect sorting to the apical mem brane is not clear. The polyimmunoglobulin receptor (plgR) is one of the be st studied proteins that follow the transcytotic pathway. It is normally de livered from the TGN to the basolateral surface of polarized Madin-Darby Ca nine Kidney (MDCK) cells from where it transports dlgA or dlgM to the apica l surface. We have studied the intracellular trafficking of plgR in Fischer rat thyroid cells (FRT), and have investigated the sorting machinery invol ved in transcytosis of this receptor in both FRT and MDCK cells. We found t hat, in contrast with MDCK cells, a significant amount (similar to 30%) of plgR reaches the apical surface by a direct pathway. Furthermore, in both c ell lines it does not associate with Triton X-100 insoluble microdomains, s uggesting that at least in these cells 'rafts' are not involved in basolate ral to apical transcytosis.