Light-regulated localization of the beta-subunit of Gq-type G-protein in the crayfish photoreceptors.

In crayfish photoreceptor cells, Gq-type G-protein plays a central role in the phototransduction pathway, and the translocation of Gq alpha has been proposed as one of the molecular mechanisms to control photoreceptor sensitivity. We here investigated beta subunit of Gq and its localization profiles under various light conditions in the crayfish photoreceptor cells to understand the functional characteristic of visual Gq in the phototransduction pathway. An immunoprecipitation experiment was performed using an anti-Gq alpha antibody and a thiol-cleavable crosslinker. A 39 kDa protein was co-immunoprecipitated with Gq alpha, but not by irradiation, in the presence of GTP gamma S. The partial amino acid sequence of the 39 kDa protein was similar to G beta e in Drosophila photoreceptors, indicating that the crayfish G beta which combines with Gq alpha is a G beta e homologue. Immunohistochemical and immunoblot analyses revealed that the amount of the G beta decreased in the rhabdomeric membranes and increased in the cytoplasm in the light, compared with that in the dark. The profile of the translocation was similar to that reported for Gq alpha. Since both alpha and beta gamma subunits are necessary for G-proteins to be activated by rhodopsin in the rhabdom, the light-modulated translocation of a G beta e homologue possibly controls the amount of Gq which can be activated by light-stimulated rhodopsin.