Determination of the Border between the Transmembrane and Cytoplasmic Domains of Human Integrin Subunits

In this study we have determined the position of the C-terminal end of the transmembrane domains of human integrin subunits (α2, α5, β1, β2) in microsomal membranes using the glycosylation mapping technique. In contrast to the common view, the transmembrane helices were found to extend roughly to Phe¹¹²⁹ in α2, to Phe¹⁰²⁶ in α5, to Ile⁷⁵⁷ in β1, and to His⁷²⁸ in β2. The α-carbon of the conserved lysine present near the C-terminal end of the transmembrane helix (Lys¹¹²⁵ in α2, Lys¹⁰²² in α5, Lys⁷⁵² in β1, and Lys⁷²⁴ in β2) is buried in the plasma membrane, and the charged amino group most likely reaches into the polar head-group region of the lipid bilayer. A possible role for the conserved lysine in integrin function is discussed.