TRANSLOCATION OF MACROMOLECULES ACROSS MEMBRANES AND THROUGH AQUEOUS CHANNELS: Translocation across membranes

摘要

Maintaining the permeability barrier of membranes is essential for cell viability. Cells allow the passage of small hydrophilic ions across the membrane by selectively gating transmembrane aqueous pores. Over the past decade an increasing amount of evidence supports a model for which transport of larger molecules such as proteins, DNA and phage also occurs through the selective gating of transmembrane aqueous channels. Studies are presented on the mechanisms by which proteins are translocated across the endoplasmic reticulum and E. coli plasma membrane through transmembrane protein-conducting channels. The model of a protein-conducting channel raises a number of challenging questions. First, if proteins are moving across through a transmembrane aqueous channel, then what is moving the protein across the membrane? Second, membrane proteins are synthesized using the same membrane-bound machinery as is used for translocating proteins across the membranes. Thus, how could a transmembrane protein-conducting channel account for the biogenesis of proteins which themselves must eventually end up spanning the bilayer multiple times? The evidence for protein-conducting channels is reviewed, and the mechanism(s) proposed for transport will be evaluated. The experiments also focus on tests for the role of aqueous channels for the biogenesis of membrane proteins.