EFFECT OF THE C-TERMINAL PROLINE REPEATS ON ORDERED PACKING OF SQUID RHODOPSIN AND ITS MOBILITY IN MEMBRANES

Negative stain electron microscopy and saturation transfer electron sp in resonance spectroscopy have been used to compare the lattice orderi ng and in-plane membrane mobility of full-length and C-terminally clea ved squid rhodopsin. The C-terminus of squid rhodopsin contains a nega tively charged region followed by 9-10 repeats of a proline-rich seque nce, not found in rhodopsins other than those of cephalopod invertebra tes, but similar proline repeats are found in other, unrelated membran e proteins. We find that the proline repeats cluster the rhodopsins in to small groups, interfering with two-dimensional crystallization and maintaining their mobility in the membrane.