Densin-180 is not a transmembrane protein.

In the central nervous system, densin-180 (densin) is one of the major components of the post-synaptic density (PSD) of excitatory synapses. Through its intricate interaction with various post-synaptic proteins, this scaffold protein may play a key role in synaptic regulation. Initial structural analyses suggest that densin is a transmembrane protein and may participate in cell-adhesion function between pre- and post-synaptic membranes. Whereas recent biochemical and mass spectrometry studies indicate that densin may instead be a membrane-associated protein with no extracellular domain. To further investigate the structural topology of densin, we began with examining the extracellular accessibility of multiple epitopes in densin. We have provided immunofluorescence evidence showing that none of the tested epitope sites in densin was accessible to extracellularly applied antibodies. In addition, both protease digestion and surface biotinylation data failed to affirm the presence of extracellular domain for densin. However, protein extraction experiments indicated that densin exhibited a significant hydrophobic interaction with the cell membrane that was not expected of cytosolic proteins. Our data therefore do not support the transmembrane model, but rather are consistent with the idea that the topology of densin involves the membrane association configuration.