Researchers have identified and sequenced the genes encoding proteins associated with nuclear pores in both yeast and vertebrates. They have used these data to determine the amino acid sequences of these proteins and have applied molecular modeling techniques to predict the three-dimensional structures of the proteins. All contain nearly identical folding regions that may serve to hold lipid membranes in sharp curves. Because this finding has been demonstrated in both yeast and vertebrates (representing organisms that diverged long ago in evolutionary history), it provides evidence of a possible mechanism for the evolution of a subcellular structure. Which subcellular structure is this