SRP-dependent co-translational translocation into the ER

Most secretory and membrane proteins enter the endoplasmic reticulum while they are being made. The signal recognition particle (SRP) binds the signal sequence or first transmembrane segment as it leaves the ribosome, the SRP receptor at the ER membrane captures the SRP-ribosome complex, and the two GTPases hand the ribosome to the Sec61 channel, through which the chain is threaded into the lumen or released laterally into the membrane. This module is the eukaryote-wide protein core of that pathway, written so that it can be evaluated in any genome.

Eukaryote-wide scope: PAINT places the nodes for SRP54, SRP19, SRP68, SRP72, the SRP receptor subunits and Sec61 alpha at Eukaryota (taxon:2759), or at an untaxoned node above it whose seeds span several kingdoms; each node is shown to be an ancestor of the human protein by its IBA rows or its IBD seeds. Read as a satisfiability rule, every non-optional part is expected in any eukaryote. Optional: the SRP9/SRP14 Alu domain, which pauses elongation and is not essential for targeting. Molecular functions come from the human gene reviews where one exists (SRP19, SRP68, SRP72, SRbeta, Sec61 gamma) and otherwise from the node's F IBD (SRP54, SRalpha, Sec61 alpha). Selector caveats are on each family: plastid paralogs can satisfy SRP54 and Sec61 alpha in plants and algae, SRbeta's subfamily also holds Arf-like GTPases, and no plant SRalpha subfamily is listed locally, so plant genomes may fail that step for want of a selector rather than of the protein. Not modelled: the SRP RNA (7SL/scR1), which is required but is not a protein; Sec61 beta, which has no local PANTHER entries; the post-translational Sec62/Sec63 route; and the TRAP, OST and EMC complexes that act at the translocon. The Drosophila-only modules dmel_sec61_translocon (which includes Sec61 beta), dmel_srp_cotranslational_targeting and dmel_er_membrane_protein_complex cover several of these in one species.