POLARIZED DISTRIBUTION AND DELIVERY OF PLASMA-MEMBRANE PROTEINS IN THYROID FOLLICULAR EPITHELIAL-CELLS

Thyroid follicular cells coordinate several oppositely located surface enzyme activities. Recent studies have raised questions about the bas ic mechanisms used to achieve thyroid surface polarity. We investigate d these mechanisms in primary thyroid epithelial monolayers cultured o n porous filters. In the steady state, most Na+/K+-ATPase and aminopep tidase N were available for surface biotinylation, and these proteins exhibited physiological distributions (basolateral and apical, respect ively). Glycosylphosphatidylinositol-anchored proteins were also apica lly distributed. By pulse-chase, newly synthesized transmembrane prote ins exhibited polarized surface delivery that was oriented similarly t o that observed at steady state. Little time elapsed between acquisiti on of Golgi-specific processing and cell surface arrival. Interestingl y when either newly synthesized or steady state-labeled thyroid peroxi dase was similarly analyzed, only similar to 30% of the enzyme was eve r detected at the cell surface. Of this, the majority was localized ap ically. The data suggest that most thyroid peroxidase remains intracel lular in these monolayers, consistent with the possibility of intracel lular iodination activity in addition to apical extracellular iodinati on. Nevertheless, in filter-polarized thyrocytes, most newly synthesiz ed plasma membrane proteins appear to be sorted in the Golgi complex f or direct delivery to apical and basolateral domains.