Saturation of, and competition for entry into, the apical secretory pathway

To investigate mechanisms of apical sorting in the secretory pathway of epi thelial cells, we expressed varying amounts of the 165 amino acid isoform o f vascular endothelial growth factor (VEGF(165)) and transforming growth fa ctor beta 1 (TGF-beta 1) via replication defective adenoviruses, Apical sor ting of both proteins was efficient at low expression levels but saturated or was reversed at high expression levers. High expression levels of TGF-be ta 1 were effective at competing VEGF(165) out of the apical pathway; howev er. VEGF(165) did not compete out TGF-beta 1. Tunicamycin inhibition experi ments showed that the apical polarity of VEGF(165) was independent of N-gly cosylation. We conclude that the apical sorting of these two molecules is a saturable, signal-mediated process, involving competition for apical sorti ng receptors. The sorting of the two proteins does not appear to involve N- glycans as sorting signals, or lectin sorters. The observations are particu larly relevant to gene therapy because they demonstrate that overexpression of a transgene can result in undesirable missorting of the encoded protein .