DIFFERENTIAL TARGETING OF RECOMBINANT FIBRONECTINS IN ATT-20 CELLS BASED ON THEIR EFFICIENCY OF AGGREGATION

In pituitary-derived AtT-20 cells, recombinant fibronectin containing the N-terminal matrix assembly domain and the C-terminal half of fibro nectin does not follow the regulated secretory pathway but instead con centrates in distinct organelles prior to secretion. These organelles are larger than the dense-core granules and localize to the cell body at sites that differ from lysosomes, endosomes and endoplasmic reticul um, Unlike the dense-core granules, their discharge is not stimulated by 8-bromo-cyclic-AMP or phorbol esters, The kinetics of intracellular transport and secretion of the recombinant fibronectin suggest that i t is present in a post-Golgi pool that turns over more slowly than con stitutive vesicles, Indeed, the fibronectin-containing organelles disa ppear with a half-time of 3 hours after inhibiting protein synthesis, Presence of the organelles correlates with intracellular aggregation o f dimeric fibronectin polypeptides. The organelles are absent in cells expressing monomeric recombinant fibronectin (lacking C-terminal dime rization sites) or the C-terminal half of fibronectin (which dimerizes but lacks the N-terminal matrix assembly domain), both of which aggre gate less efficiently than dimeric fibronectin. Instead, the latter po lypeptides enter the dense-core granules. Thus while the formation of the fibronectin-containing organelles may require efficient aggregatio n, it may not require a specific structural signal, Moreover, efficien t aggregation is not necessarily a prerequisite for following the regu lated pathway.