Stitching proteins into membranes, not sew simple

Abstract

Most integral membrane proteins located within the endomembrane system of eukaryotic cells are first assembled co-translationally into the endoplasmic reticulum (ER) before being sorted and trafficked to other organelles. The assembly of membrane proteins is mediated by the ER translocon, which allows passage of lumenal domains through and lateral integration of transmembrane (TM) domains into the ER membrane. It may be convenient to imagine multi-TM domain containing membrane proteins being assembled by inserting their first TM domain in the correct orientation, with subsequent TM domains inserting with alternating orientations. However a simple threading model of assembly, with sequential insertion of one TM domain into the membrane after another, does not universally stand up to scrutiny. In this article we review some of the literature illustrating the complexities of membrane protein assembly. We also present our own thoughts on aspects that we feel are poorly understood. In short we hope to convince the readers that threading of membrane proteins into membranes is 'not sew simple' and a topic that requires further investigation.

Viroporins, Examples of the Two-Stage Membrane Protein Folding Model

“…Due to the similar size and membrane architecture between class I viroporins (see above) it is generally assumed that all of them use the co-translational pathway in their way to the ER membrane. The different topologies found between subclasses IA and IB would arise from rearrangements of the nascent chain while locating within the translocon (see [ 33 ] for a recent review).…”

“…Next, the lateral crack expands across the entire channel creating a lateral gate facing the core of the membrane [ 45 ]. At this stage the protein sequence within the translocon will, depending on its characteristics, laterally partition into the hydrophobic core of the membrane (in the case of TM segments) [ 33 ] or, if the polypeptide region is not hydrophobic enough, continue through the channel to be translocated into the ER lumen.…”

Membrane insertion and topology of the translocon-associated protein (TRAP) gamma subunit

“…The ribosome and the nascent chain are then transferred to the translocon, a multi-protein complex that facilitates insertion of integral membrane proteins into the lipid bilayer and translocation of secreted proteins across the lipid bilayer [1]. Translocons are not passive pores in the bilayer, but instead are dynamic complexes that cycle between ribosome-bound and ribosome-free states, and convert between translocation and membrane integration modes of action, while maintaining the membrane permeability barrier [2,3]. The core components of the mammalian translocon are the Sec61 α, β and γ subunits [4] and the translocating chain-associating membrane protein [5,6].…”

NMR Investigation of Structures of G-protein Coupled Receptor Folding Intermediates

“…In order to traffic proteins to membranes in most cells the signal-recognition particle targets the nascent chains emerging from the ribosome tunnel to the translocon complex (13 (12,(18)(19)(20). Folding of polytopic membrane proteins is the result of a series of events that include helixinsertion into the hydrophobic core and sequestering of loop-sequences into cytosolic or extracellular space (11,21). The timing of the chain insertion and the localization of TM helices would be expected to be a consequence of the amino acid sequence and the interaction of the growing polypeptide chain with the membrane.…”

“…Considering that proteins are synthesized starting with the Nterminus of the polypeptide chain, TM1 is expected to be the first segment that is inserted into the hydrophobic core, followed by TM2 and so on (21). We therefore report on studies of polypeptides corresponding to the overlapping fragments TM1, TM1-TM2 (TM12) and TM1-TM2-TM3 (TM123) (Fig.…”