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

Abstract

To investigate mechanisms of apical sorting in the secretory pathway of epithelial cells, we expressed varying amounts of the 165 amino acid isoform of vascular endothelial growth factor (VEGF 165) and transforming growth factor ␤1 (TGF-␤1) via replication defective adenoviruses. Apical sorting of both proteins was efficient at low expression levels but saturated or was reversed at high expression levels. High expression levels of TGF-␤1 were effective at competing VEGF 165 out of the apical pathway; however, VEGF165 did not compete out TGF-␤1. Tunicamycin inhibition experiments showed that the apical polarity of VEGF165 was independent of N-glycosylation. We conclude that the apical sorting of these two molecules is a saturable, signal-mediated process, involving competition for apical sorting receptors. The sorting of the two proteins does not appear to involve N-glycans as sorting signals, or lectin sorters. The observations are particularly relevant to gene therapy because they demonstrate that overexpression of a transgene can result in undesirable missorting of the encoded protein.

Cholesterol depletion induces PKA-mediated basolateral-to-apical transcytosis of the scavenger receptor class B type I in MDCK cells

“…Apical sorting is promoted by information held by exoplasmic or cytosolic domains, or by peptidic or glycosylphosphatidylinositol membrane anchors (1) and can be entirely dependent on proteinaceous sorting features (1,15,24,25) although, for yet unknown reasons, it frequently requires N-and͞or Oglycosylation (1,26), suggesting multiple decoding systems. In contrast, basolateral sorting signals are restricted to relatively short linear sequences located in the cytosolic domain of transmembrane proteins (27), and are decoded by cytosolic adaptor proteins (28,29).…”

Apical sorting of a voltage- and Ca 2+ -activated K + channel α-subunit in Madin-Darby canine kidney cells is independent of N-glycosylation

“…Using a viral model protein, we previously have described the existence of an apical secretory pathway mediated by proteinaceous instead of glycan sorting signals (36). More recent evidence has suggested that glycan-mediated and glycan-independent apical sorting mechanisms of secreted proteins compete for a nonlectin apical sorting receptor (50). Because several proteins require N-or O-glycosylation for apical delivery, glycans could act as structural facilitators or stabilizers of apical information encoded in protein conformations (1).…”

Sublytic Membrane-Attack-Complex (MAC) Activation Alters Regulated Rather than Constitutive Vascular Endothelial Growth Factor (VEGF) Secretion in Retinal Pigment Epithelium Monolayers

“…Ref. 29,32) via a saturable process (57). Regulated secretion of polypeptides present in granular vesicles occurs via exocytosis triggered by Ca 2ϩ .…”