We have investigated the distribution of the transmembrane myelin proteolipid protein when inserted into an oil-swollen lamellar phase, stabilized by steric interactions. When the hydrophobic membrane thickness, D, is larger than the hydrophobic length of the protein, d_π, only repulsive interactions are found between proteins. The repulsive forces are of electrostatic nature, arising from charges carried by the protein. The interaction potential between proteins, deduced from digitized freeze-fracture micrographs, is well fitted when the classical screened electrostatic model is used. When D is smaller than d_π, an attractive force is observed in addition to the repulsive electrostatic interactions. The attractive force originates from the membranes fluctuations. The model of membrane-mediated interactions due to the membrane thermal undulations permits us to describe our results when used in combination with the electrostatic potential.