Quality Control of Integral Membrane Proteins by Assembly-Dependent Membrane Integration

Abstract

Summary Cell surface multi-protein complexes are synthesized in the endoplasmic reticulum (ER) where they undergo co-translational membrane integration and assembly. The quality control mechanisms that oversee these processes remain poorly understood. We show that less hydrophobic transmembrane (TM) regions derived from several single-pass TM proteins can enter the ER lumen completely. Once mislocalized, they are recognized by the Hsp70 chaperone BiP. In a detailed analysis for one of these proteins, the αβT cell receptor (αβTCR), we show that unassembled ER-lumenal subunits are rapidly degraded, whereas specific subunit interactions en route to the native receptor promote membrane integration of the less hydrophobic TM segments, thereby stabilizing the protein. For the TCR α-chain, both complete ER import and subunit assembly depend on the same pivotal residue in its TM region. Thus, membrane integration linked to protein assembly allows cellular quality control of membrane proteins and connects the lumenal ER chaperone machinery to membrane protein biogenesis.

The Influenza Virus Neuraminidase Protein Transmembrane and Head Domains Have Coevolved

“…The functional roles of TMD interactions in single-spanning eukaryotic membrane proteins have been linked mainly to homoand heterocomplex assembly (5,6,9,46,47). Here, we found that the TMD contributes to the folding efficiency of the distal N1 head domain in a temperature-dependent manner.…”

“…Instead, studies with model and natural TMD segments in reductionist systems have been used to determine how they interact (reviewed in references 1-3). The biological functions of these TMD interactions in bitopic proteins are much less characterized and have been linked mainly to oligomerization and activity (4)(5)(6)(7)(8)(9). The current challenge is to better define the plasticity of bitopic TMD interactions and how they contribute to protein folding and activity in native biological systems.…”

The Large Hsp70 Grp170 Binds to Unfolded Protein Substrates in Vivo with a Regulation Distinct from Conventional Hsp70s

“…Many ER proteins are glycosylated on asparagine residues within Asn-X-Ser/Thr sequons (1,3). It has been reported that the T cell receptor ␤-chain (TCR␤) populates two isoforms that vary in their degree of glycosylation and which are readily distinguished by SDS-PAGE (28,36). Although the majority of TCR␤ is glycosylated at two sites co-translationally, a species can be detected that is only glycosylated on a single site 35 S]cysteine/methionine for 1 h and chased for 1 h to allow maturation of the chaperones before lysing either in the presence of ATP or apyrase.…”

Dimerization-dependent Folding Underlies Assembly Control of the Clonotypic αβT Cell Receptor Chains

“…The ␣␤TCR is a longstanding model for the quality control of oligomeric membrane proteins, and a great deal is understood about cellular checkpoints in its assembly (7)(8)(9)(10)(11)(12)(13)(14)(15). It is composed of eight polypeptide chains, the clonotypic ␣ and ␤ chains and the invariant co-receptor chains (CD3␥, ␦, ⑀, and ; Fig.…”

“…First, the ␣ and ␤ chains pair with their designate CD3 co-receptor subunits, giving rise to ␣-CD3␦⑀ and ␤-CD3␥⑀ trimers, respectively (19). In each case, complementary basic/acidic residues in the transmembrane segments of these chains guide assembly of the trimers and, as has been dissected in detail for the ␣ chains, provide a means to identify chains that do not assemble with CD3 components (7,8,10,12,20). Retention motifs in the CD3 subunits appear to play an important role in the final step of ␣␤TCR assembly: interaction of ␣␤-CD3␥␦⑀ 2 hexamers with 2 homodimers to form the complete ␣␤TCR (21,22).…”