Rhodopsin is the G-protein-coupled membrane receptor that initiates the visual transduction cascade in retinal photoreceptors. In the present study rhodopsin from the dark-adapted retinas of squid (Loligo forbesi) was detergent-extracted, purified, and reconstituted into native squid photoreceptor lipids following proteolytic cleavage of its proline-rich C-terminus. two-dimensional crystals of C-terminally truncated rhodopsin reconstituted from octyl glucoside solution formed in a p222(1) lattice (a = 44 Angstrom, b = 131 Angstrom). Electron micrographs of frozen-hydrated crystals were processed and a projection structure to 8 Angstrom resolution was calculated. The projection map obtained is very similar to maps previously determined for bovine and frog rhodopsins although the crystal packing of the molecules is quite different. Comparison of the maps shows that the arrangement of ol-helices in the proteins is very similar despite their great phylogenetic distance; this structure is likely to be present in the whole superfamily of G-protein-coupled receptors. Invertebrate rhodopsins have a large insertion in the helix 5-helix 6 loop. Assignment of an additional density in the squid rhodopsin map to this region supports a previously proposed helix assignment and identifies the end-to-end contacts as helices 1 and 5. (C) 1996 Academic Press, Inc.