Protein-Protein Interactions

Abstract

Proteins exist in the cellular aqueous environment containing carbohydrate, lipid, nucleic acids and other proteins. While soluble proteins interact with their substrate in a three-dimensional matrix, a number of the cellular proteins exist in a membranous structure, located as components of one or another of the cellular membranes. Movement of these proteins is more restricted, and involves two-dimensional or translational mobility along the membrane. As one might expect, restriction of some proteins to a two-dimensional matrix would serve the purpose of facilitating their interactions. Indeed, a number of enzymes which function by interaction with other proteins, most notably electron transfer proteins, are membrane bound. One example of such enzymes are those serving the electron transfer pathways of the inner mitochondrial membrane. A second example would be the proteins localized to the endoplasmic reticulum and comprising a number of electron transfer oxidative pathways. Such proteins and their enzymatic activities are usually studied in the membrane fragments, e.g., the endoplasmic reticulum, the vesicular particles called microsomes (Claude 1943). The microsomes contain several electron transfer chains, all of which, parenthetically, may interact with the microsomal electron transfer hemoprotein cytochrome b ₅ (Schenkman et al. 1976). These enzymes include the cytochrome P450 monooxygenases (Jansson et al. 1985 and references therein), the stearoyl CoA Δ9 desaturase (Strittmatter et al. 1974), the linolenoyl CoA Δ6 desaturase (Okayasu et al. 1981), the γ-linolenoyl CoA Δ5 desaturase (Do and Sprecher 1975), and the fatty acid elongase (Keyes et al. 1979).