This family represents Asterix proteins which are a component of the PAT complex, an endoplasmic reticulum (ER)-resident membrane multiprotein complex that facilitates multi-pass membrane proteins insertion into membranes [1]. The PAT complex acts as an intramembrane chaperone by directly interacting with nascent transmembrane domains (TMDs), releasing its substrates upon correct folding, and is needed for optimal biogenesis of multi-pass membrane proteins [1]. Asterix is the substrate-interacting subunit of the PAT complex and associates with the first TMD of the nascent chain. The PAT complex favors the binding to TMDs with exposed hydrophilic amino acids within the lipid bilayer and provides a membrane-embedded partially hydrophilic environment in which TMD1 binds [1].

1: An intramembrane chaperone complex facilitates membrane protein biogenesis. Chitwood PJ, Hegde RS; Nature. 2020;584:630-634. PMID:32814900