1. Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
2. Department of Ophthalmology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
3. Howard Hughes Medical Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
4. Department of Biochemistry and Molecular Biology, and Department of Ophthalmology, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3

Correspondence to: Robert S. Molday²King-Wai Yau^1,3,4 Correspondence and requests for materials should be addressed to R.S.M. (e-mail: Email: molday@unixg.ubc.ca). or K.-W.Y. (e-mail: Email: kyau1@jhem.jhmi.edu).

Abstract

Cyclic nucleotide-gated (CNG) channels are crucial for visual and olfactory transductions^{1, 2, 3, 4}. These channels are tetramers and in their native forms are composed of A and B subunits⁵, with a stoichiometry thought to be 2A:2B (refs 6, 7). Here we report the identification of a leucine-zipper⁸-homology domain named CLZ (for carboxy-terminal leucine zipper). This domain is present in the distal C terminus of CNG channel A subunits but is absent from B subunits, and mediates an inter-subunit interaction. With cross-linking, non-denaturing gel electrophoresis and analytical centrifugation, this CLZ domain was found to mediate a trimeric interaction. In addition, a mutant cone CNG channel A subunit with its CLZ domain replaced by a generic trimeric leucine zipper produced channels that behaved much like the wild type, but less so if replaced by a dimeric or tetrameric leucine zipper. This A-subunit-only, trimeric interaction suggests that heteromeric CNG channels actually adopt a 3A:1B stoichiometry. Biochemical analysis of the purified bovine rod CNG channel confirmed this conclusion. This revised stoichiometry provides a new foundation for understanding the structure and function of the CNG channel family.