What property of membrane proteins contributes to their low solubility in water?

The low solubility of membrane proteins in water is attributed to the presence of long stretches of nonpolar amino acids within their structure. These nonpolar regions create hydrophobic characteristics that interact favorably with the lipid bilayer of cell membranes but are not compatible with the polar environment of water. As a result, the long hydrophobic sequences limit the ability of the protein to dissolve in aqueous solutions, as nonpolar substances tend to separate from water rather than mix with it.

In contrast, the presence of hydrophilic amino acids would typically enhance solubility in water due to their attraction to water molecules. Short stretches of nonpolar amino acids, while still contributing to hydrophobic interactions, would not provide the same degree of incompatibility with water as longer sequences. The incorporation of phospholipids pertains to the membrane's overall structure and its ability to form bilayers, rather than the solubility characteristics of the proteins embedded within those membranes. Thus, the correct choice highlights the critical role of long nonpolar amino acid stretches in conferring low solubility in water.