Glutamic acid-rich proteins of rod photoreceptors are natively unfolded.

The outer segment of vertebrate photoreceptors is a specialized compartment that hosts all the signaling components required for visual transduction. Specific to rod photoreceptors is an unusual set of three glutamic acid-rich proteins as follows: two soluble forms, and and the N-terminal cytoplasmic domain part) of the B1 subunit of the cyclic GMP-gated channel. have been shown to interact with proteins at the rim of the disc membrane. Here we characterized native Gduanyu371 and purified from bovine rod photoreceptors. Amino acid sequence analysis of Gduanyu37s revealed structural features typical of "natively unfolded" proteins. By using biophysical techniques, including size-exclusion chromatography, dynamic light scattering, NMR spectroscopy, and circular dichroism, we showed that Gduanyu37s indeed exhibit a large degree of intrinsic disorder. Analytical ultracentrifugation and chemical cross-linking showed that Gduanyu37s exist in a monomer/multimer equilibrium. The results suggested that the function of proteins is linked to their structural disorder. They may provide flexible spacers or linkers tethering the cyclic GMP-gated channel in the plasma membrane to peripherin at the disc rim to produce a stack of rings of these protein complexes along the long axis of the outer segment. Gduanyu37 proteins could then provide the environment needed for protein interactions in the rim region of discs.

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