Evolution and adaptation of single-pass transmembrane proteins.

A comparative analysis of 6039 single-pass (bitopic) membrane proteins from six evolutionarily distant organisms was performed based on data from the Membranome database. The observed repertoire of bitopic proteins is significantly enlarged in eukaryotic cells and especially in multicellular organisms due to the diversification of enzymes, emergence of proteins involved in vesicular trafficking, and expansion of receptors, structural, and adhesion proteins. The majority of bitopic proteins in multicellular organisms are located in the plasma membrane (PM) and involved in cell communication. Bitopic proteins from different membranes significantly diverge in terms of their biological functions, size, topology, domain architecture, physical properties of transmembrane (TM) helices and propensity to form homodimers. Most proteins from eukaryotic PM and endoplasmic reticulum (ER) have the N-out topology. The predicted lengths of TM helices and hydrophobic thicknesses, stabilities and hydrophobicities of TM α-helices are the highest for proteins from eukaryotic PM, intermediate for proteins from prokaryotic cells, ER and Golgi apparatus, and lowest for proteins from mitochondria, chloroplasts, and peroxisomes. Tyr and Phe residues accumulate at the cytoplasmic leaflet of PM and at the outer leaflet of membranes of bacteria, Golgi apparatus, and nucleus. The propensity for dimerization increases from unicellular to multicellular eukaryotes, from enzymes to receptors, and from intracellular membrane proteins to PM proteins. More than half of PM proteins form homodimers with a 2:1 ratio of right-handed to left-handed helix packing arrangements. The inverse ratio (1:2) was observed for dimers from the ER, Golgi and vesicles.