Size comparisons among integral membrane transport protein homologues in Bacteria, Archaea, and Eucarya

Integral membrane proteins from over 20 ubiquitous families of channels, se condary carriers, and primary active transporters were analyzed for average size differences between homologues from the three domains of life: Bacter ia, Archaea, and Eucarya, The results showed that while eucaryotic homologu es are consistently larger than their bacterial counterparts, archaeal homo logues are significantly smaller. These size differences proved to be due p rimarily to variations in the sizes of hydrophilic domains localized to the N termini, the C termini, or specific loops between transmembrane alpha -h elical spanners, depending on the family. Within the Eucarya domain, plant homologues proved to be substantially smaller than their animal and fungal counterparts. By contrast, extracytoplasmic receptors of ABC-type uptake sy stems in Archaea proved to be larger on average than those of their bacteri al homologues, while cytoplasmic enzymes from different organisms exhibited little or no significant size differences. These observations presumably r eflect evolutionary pressure and molecular mechanisms that must have been o perative since these groups of organisms diverged from each other.