Practice question

Integral membrane proteins are defined by hydrophobic transmembrane segments, typically 20-25 residues forming alpha helix crossing hydrocarbon core stabilized by van der Waals and hydrogen bonding, or beta-barrel of multiple antiparallel strands as in VDAC. Extraction requires detergent disrupting bilayer rather than high salt. Aquaporin family represents classic multipass integral protein: tetramer in plasma membrane where each monomer with six transmembrane helices forms hourglass pore lined by NPA motifs Asn-Pro-Ala that orient water molecules in single file and aromatic-arginine constriction selecting against protons, facilitating rapid osmotic water equilibration essential in kidney proximal tubule, collecting duct regulated by vasopressin, and astrocyte edema control. Spectrin is peripheral cytoskeletal tetramer binding actin via adducin and band 3 through ankyrin on inner leaflet, easily eluted with low ionic strength. Histone octamer wraps DNA in nucleus, tubulin forms cytosolic microtubules of 25 nm. Thus aquaporin exemplifies true transmembrane embedding with functional pore within bilayer, while others are soluble or peripheral.