Small Residues Inhibit Homo-Dimerization of the Human Carbonic Anhydrase XII Transmembrane Domain

Amino acids with small side chains and motifs of small residues in a distance of four are rather abundant in human single-span transmembrane helices. While interaction of such helices appears to be common, the role of the small residues in mediating and/or stabilizing transmembrane helix oligomers remains mostly elusive. Yet, the mere existence of (small)xxx(small) motifs in transmembrane helices is frequently used to model dimeric TM helix structures. The single transmembrane helix of the human carbonic anhydrases XII contains a large number of amino acids with small side chains, and critical involvement of these small amino acids in dimerization of the transmembrane domain has been suggested. Using the GALLEX assay, we show here that the transmembrane domain indeed forms a strong transmembrane helix oligomer within a biological membrane. However, single or multiple mutations of small residue(s) to isoleucine almost always increased, rather than decreased, the interaction propensities. Reduction of helix flexibility and of protein-lipid contacts caused by a reduced lipid accessible surface area likely results in stabilization of helix-helix interactions within the membrane.

Automated summary

Human single-span transmembrane helices often contain small residues and motifs, playing an important role in mediating and stabilizing helix oligomers. Despite the common interaction of these helices, the exact role of small residues in helix dimerization remains unclear. Experimental evidence suggests that mutations of small residues to isoleucine actually enhance helix-helix interactions within the membrane, possibly due to reduced helix flexibility and protein-lipid contacts.