RETINAL rods respond to light with a membrane hyperpolarization produced by a G-protein-mediated signalling cascade that leads to cyclic GMP hydrolysis and the consequent closure of a cGMP-gated channel that is open in darkness 1-7. A protein that forms this channel has recently been purified from bovine retina8 and molecularly cloned9, suggesting that the native cGMP-gated channel might be a homo-oligomer10. Here we report the cloning of another protein from human retina which has only about 30% overall identity to the rod channel subunit. This protein, immunocytochemically localized to rod outer segments, does not form functional channels by itself. However, when co-expressed with the cloned human rod channel protein11, it introduces rapid flickers to the channel openings that are characteristic of the native channel12-16. The hetero-oligomeric channel is also highly sensitive to the blocker L-cis-diltiazem, like the native channel15,17,18. This new protein thus seems to be another subunit of the native rod channel. The hetero-oligomeric nature of the rod channel means that it is no exception to a common motif shared by other ligand-gated channels.
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