Overview of Membrane Protein Assembly Pathways in Prokaryotic and Eukaryotic Cells

Overview of Membrane Protein Assembly Pathways in Prokaryotic and Eukaryotic Cells 

Over the past 30 years, the study of intracellular protein sorting has grown to a large and diversified field. A host of different sorting pathways have been found in both prokaryotic and eukaryotic cells, and most of these can handle both soluble and membrane-bound proteins. 

The most well understood pathway is the one that delivers secretory and membrane proteins to the endoplasmic reticulum (ER) membrane in eukaryotic cells and to the inner membrane in bacteria. In both kinds of cells, the pivotal role is played by the so-called Sec61 (in eukaryotes) or SecYEG (in prokaryotes) translocon, a multisubunit translocation channel that provides a conduit for soluble proteins to cross the membrane. The same translocon also serves to integrate membrane proteins into the lipid bilayer. 

Bacteria harbor additional inner membrane translocation machineries, such as the twin-arginine translocation (Tat) system, the YidC system, and the so-called type II translocation systems that are dedicated to one or a small number of substrates and ensure their delivery into the extracellular medium. 

The Tat and YidC systems are also found in certain organelles in eukaryotic cells. The former is present in the thylakoid membrane in plant cells, and the latter is found both in the thylakoid membrane (where it is called the Albino3 system) and in the inner mitochondrial membrane (where it is called the Oxalp system). Both chloroplasts and mitochondria appear to have unique systems for importing proteins across their outer and inner membranes, and these systems also handle membrane proteins. 

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