The phototransduction cascade in invertebrates involves the coupling o
f rhodopsin activation to the action of the enzyme phospholipase C. Th
is step is performed by G-proteins. An antibody against the cu-subunit
of a mouse G(q) type G-protein recognized protein bands in Western bl
ots of lateral eye and ventral nerve photoreceptors of Limulus. The pr
otein bands had an apparent molecular mass of about 42 kDa. The antibo
dy also recognized protein bands of a similar molecular mass in immuno
blots of brain and intestine tissue. Immunoreactivity was found in lat
eral eye frozen sections where it was confined to the rhabdom region.
When the antibody was applied to ultrathin sections of ventral nerve p
hotoreceptors, the highest density of labeling was found on the rhabdo
meral microvilli, but gold particles were also scattered in the cytopl
asm. We conclude that a G-protein of the type G(q) participates in the
phototransduction of Limulus.