transmembrane protein

Meaning: A transmembrane protein passes through the bilayer once (single-pass) or several times (multipass). The segment inside the membrane is rich in nonpolar residues such as leucine, valine, isoleucine, and phenylalanine, which interact with the fatty acid tails by hydrophobic interactions. Most spans are α-helices of roughly 20 residues. Porins in bacterial and mitochondrial outer membranes use β-barrels instead. Polar and charged residues sit in the loops and tails exposed to water, or line the inside of a channel or pore. Examples include ion channels, transporters, G protein-coupled receptors (seven helices), receptor tyrosine kinases, and integrins.

On the MCAT: A passage gives an amino acid sequence or hydropathy plot, and you identify the membrane-spanning region as a stretch of nonpolar residues. You may predict that replacing one of those residues with a charged one disrupts folding or insertion. Another common item is extraction: detergents are needed to pull out transmembrane proteins, while a high-salt or pH change can release peripheral proteins. Under the fluid mosaic model, these proteins can diffuse laterally in the bilayer, but flipping from one leaflet to the other is very rare.

Common confusion: Transmembrane and integral are not quite the same. Every transmembrane protein is integral, but a few integral proteins are embedded in only one leaflet without crossing. Peripheral proteins bind the surface by weak electrostatic interactions or hydrogen bonds and do not span the membrane.