We have investigated the perturbation induced by a transmembrane protein in size, shape, and organization of reverse micelles, in a ternary system made of tetraethylene glycol monododecyl ether, dodecane, and water. The myelin proteolipid was solubilized in the micellar solution at high yield, preserving its a-helical structure. Characterization studies were carried out at a constant water-to-surfactant molar ratio of 13.7 and at a temperature of 31 degrees C, close to the L-alpha lamellar phase transition. Small-angle X-ray scattering experiments reveal that the shape of the protein-containing individual prolate micelles remains unchanged compared to protein-free micelles previously studied. The plot of In I(q) versus qR indicates an aggregation mechanism probably originating from intermicellar protein bridging. Dynamic light scattering measurements confirm attractive interactions between protein-containing micelles and provide the size of micelle-protein aggregates, which increases as a function of protein concentration. The hydrodynamic radius RH varies from 290 to 450 Angstrom when C, the protein-to-surfactant molar ratio, increases from 23 x 10(-5) to 42 x 10(-5), Correlative variations of R-G, the radius of gyration measured by static light scattering, and of RH for various values of C indicate that the novel structural arrangement is disk-oblate shaped and could be considered as a precursor of a protein-containing lamellar phase.