ANALYSIS OF TRANSPORT AND TARGETING OF SYNDECAN-1 - EFFECT OF CYTOPLASMIC TAIL DELETIONS

Madin-Darby canine kidney (MDCK) cells and Chinese hamster ovary (CHO) cells were transfected with wild-type and cytoplasmic deletion mutant s of mouse syndecan-1 to study the requirements for transport and pola rized expression of this proteoglycan. Expression in MDCK cells reveal ed that wild-type syndecan-1 is directed to the basolateral surface vi a a brefeldin A-insensitive route. A deletion of the last 12 amino aci ds of the syndecan-1 cytoplasmic tail (CT22) was sufficient to result in the appearance of mutant proteoglycans at both the basolateral and apical cell surfaces. Treatment with brefeldin A was able to prevent a pical transport of the mutants. We thus propose that the C-terminal pa rt of the cytoplasmic tail is required for steady-state basolateral di stribution of syndecan-1. In CHO cells a deletion of the last 25 or 33 amino acids of the 34-residue cytoplasmic domain (CT9 and CT1, respec tively) resulted in partial retention of the mutants in the endoplasmi c reticulum (ER). A deletion mutant lacking the last 12 amino acids (C T22) was not retained. Interestingly, the unglycosylated core proteins of the CT9 and CT1 mutants showed a significantly lower apparent mole cular weight when analyzed by sodium dodecyl sulfate (SDS) polyacrylam ide gel electrophoresis than wild-type syndecan-1. However, when CHO t ransfectants expressing the CT1 mutant were incubated with brefeldin A , causing fusion of the ER and Golgi, CT1 ran with an almost equally h igh apparent molecular weight as the wild-type molecule. This would su ggest that syndecan-1 undergoes extensive posttranslational modificati ons or forms an SDS-resistant dimer/complex after transit from the ER.