Membrane receptor trafficking: evidence of proximal and distal zones conferred by two independent endoplasmic reticulum localization signals.

The generic membrane trafficking signals of internal RXR and carboxyl-terminal KKXX motifs direct intracellular endoplasmic reticulum (ER) localization of the signal-bearing proteins. These signaling motifs play a critical role in partitioning proteins into designated subcellular compartments by functioning as an intracellular "zip code." In the process of determining the potential distinctions between these two otherwise functionally identical motifs, two functional zones of these signals were revealed. The KKXX signal was effective only when it was positioned closer to the membrane surface. In contrast, under identical conditions, the internal RXR signal was functional when it was positioned distally from the membrane. Different from the C-terminal KKXX signal, the internal RXR motif may be present in multiple copies. The receptor with multivalent RXR motifs displayed similar trafficking behavior to that of the same receptor with one copy of the RXR motif. The distinctive operating ranges from their anchored membrane surface provide experimental evidence for the notion that there are functional zoning layers within which membrane protein signal motifs are active.