THE ION-CHANNEL BEHAVIOR OF THE NUCLEAR-PORE COMPLEX

Macromolecule-conducting pores have been recently recognized as a dist inct class of ion channels. The poor role of macromolecules as electri cal charge carriers can be used to detect their movement along electro lyte-filled pores. Because of their negligible contribution to electri cal ion currents, translocating macromolecules reduce the net conducti vity of the medium inside the pore, thus decreasing the measured pore ion conductance. In the extreme case, a large translocating macromolec ule can interrupt ion flow along the pore lumen, reflected as a neglig ible pore conductance. Therefore, ion conductance serves as a measurem ent of macromolecular transport, with lesser values indicating greater macromolecular translocation (in size and/or number). Such is the pri nciple of operation of the widely used Coulter counter, an instrument for counting and sizing particles. It has long been known that macromo lecules translocate across the central channel of nuclear pore complex es (NPCs), Recently, large conductance ion channel activity (100-1000 pS) was recorded from the nuclear envelope (NE) of various preparation s and it was suggested that NPCs may be the source of this activity. D espite its significance to understanding the regulation of transcripti on, replication, mRNA export, and thus gene expression of normal and p athological states, no report has appeared demonstrating that this cha nnel activity corresponds to ion flow along the central channel of the NPC, Here we present such a demonstration in adult mouse cardiac myoc yte nuclei. In agreement with concepts introduced for macromolecule-co nducting channels, our patch clamp experiments showed that ion conduct ance is reduced, and thus that ion flow is restricted during transloca tion of macromolecules containing nuclear targeting signals. Ion flow was blocked by mAb414, a monoclonal antibody raised against a major NP C glycoprotein and known to localize on the NPC channel where it block s macromolecular transport. These results also establish patch clamp a s a useful technique for the measurement of macromolecular translocati on along the large central channel of the NPC and provide a basis for the design of future investigations of nuclear signaling for control o f gene activity, mRNA export for gene expression, as well as other pro cesses subservient to NPC-mediated nucleocytoplasmic exchange.