Architecture and dynamics of microvillar photoreceptor membranes of a cephalopod

Microvillar membranes of cephalopod photoreceptors, Eledone aldrovandii were analysed with respect to their protein and lipid composition. Molecular dynamics of this membrane type were investigated by ESR measurements using frog rod outer segment membranes as a reference system. The photoreceptor membrane is composed of about 56 wt% protein and 44 wt% lipid. Rhodopsin (mol wt 51 000) represents at least 70% of the membrane protein. The molar ratio of phospholipid to cholesterol to rhodopsin is about 55:24:1. Phosphatidylcholine (28.9 mol%) and phosphatidylethanolamine (27.8 mol%) are the major phospholipids. The ESR measurements suggest that the cephalopod photoreceptor membrane is less fluid than from rod outer segment membranes although the major phospholipids show remarkable high levels of polyunsaturated fatty acids (e.g. 88 mol% in phosphatidylethanolamine). It is concluded that the lower fluidity of microvillar membranes results in part from high cholesterol content and that a restricted mobility of rhodopsin in this membrane does not result only from the fact that the membrane is rolled into a microvillar structure.