A selectivity filter in the ER membrane protein complex limits protein misinsertion at the ER

Pleiner, Hazu, Pinton Tomaleri et al. identify a selectivity filter in the ER membrane protein complex (EMC) and show that the EMC protects the integrity of the ER proteome by limiting the misinsertion of mitochondrial tail-anchored proteins and enforcing the correct topology of multipass membrane proteins. Tail-anchored (TA) proteins play essential roles in mammalian cells, and their accurate localization is critical for proteostasis. Biophysical similarities lead to mistargeting of mitochondrial TA proteins to the ER, where they are delivered to the insertase, the ER membrane protein complex (EMC). Leveraging an improved structural model of the human EMC, we used mutagenesis and site-specific crosslinking to map the path of a TA protein from its cytosolic capture by methionine-rich loops to its membrane insertion through a hydrophilic vestibule. Positively charged residues at the entrance to the vestibule function as a selectivity filter that uses charge-repulsion to reject mitochondrial TA proteins. Similarly, this selectivity filter retains the positively charged soluble domains of multipass substrates in the cytosol, thereby ensuring they adopt the correct topology and enforcing the positive-inside rule. Substrate discrimination by the EMC provides a biochemical explanation for one role of charge in TA protein sorting and protects compartment integrity by limiting protein misinsertion.

Automated summary

A selectivity filter in the ER membrane protein complex (EMC) is identified and shown to protect the integrity of the ER proteome by limiting the misinsertion of mitochondrial tail-anchored proteins and enforcing the correct topology of multipass membrane proteins. The selectivity filter rejects mitochondrial tail-anchored proteins and retains the positively charged soluble domains of multipass substrates in the cytosol, ensuring correct topology and protecting compartment integrity.