Spf1 and Ste24: quality controllers of transmembrane protein topology in the eukaryotic cell

Donald J Tipper; Carol A Harley

doi:10.3389/fcell.2023.1220441

Spf1 and Ste24: quality controllers of transmembrane protein topology in the eukaryotic cell

Front Cell Dev Biol. 2023 Aug 3:11:1220441. doi: 10.3389/fcell.2023.1220441. eCollection 2023.

Authors

Donald J Tipper¹, Carol A Harley^{2

3}

Affiliations

¹ University of Massachusetts Medical School, Worcester, MA, United States.
² i3S-Instituto de Investigação e Inovação em Saude, Universidade do Porto, Porto, Portugal.
³ IBMC-Instituto de Biologia Molecular e Celular, Universidade do Porto, Porto, Portugal.

Abstract

DNA replication, transcription, and translation in eukaryotic cells occur with decreasing but still high fidelity. In contrast, for the estimated 33% of the human proteome that is inserted as transmembrane (TM) proteins, insertion with a non-functional inverted topology is frequent. Correct topology is essential for function and trafficking to appropriate cellular compartments and is controlled principally by responses to charged residues within 15 residues of the inserted TM domain (TMD); the flank with the higher positive charge remains in the cytosol (inside), following the positive inside rule (PIR). Yeast (Saccharomyces cerevisiae) mutants that increase insertion contrary to the PIR were selected. Mutants with strong phenotypes were found only in SPF1 and STE24 (human cell orthologs are ATP13A1 and ZMPSte24) with, at the time, no known relevant functions. Spf1/Atp13A1 is now known to dislocate to the cytosol TM proteins inserted contrary to the PIR, allowing energy-conserving reinsertion. We hypothesize that Spf1 and Ste24 both recognize the short, positively charged ER luminal peptides of TM proteins inserted contrary to the PIR, accepting these peptides into their large membrane-spanning, water-filled cavities through interaction with their many interior surface negative charges. While entry was demonstrated for Spf1, no published evidence directly demonstrates substrate entry to the Ste24 cavity, internal access to its zinc metalloprotease (ZMP) site, or active withdrawal of fragments, which may be essential for function. Spf1 and Ste24 comprise a PIR quality control system that is conserved in all eukaryotes and presumably evolved in prokaryotic progenitors as they gained differentiated membrane functions. About 75% of the PIR is imposed by this quality control system, which joins the UPR, ERAD, and autophagy (ER-phagy) in coordinated, overlapping quality control of ER protein function.

Keywords: positive inside rule; quality control; topology; topology error recognition; transmembrane proteins.

Publication types

Review

Grants and funding

This work was funded by National Funds through FCT—Fundação para a Ciência e a Tecnologia, I.P., under project UIDB/04293/2020 and FCT - 2022 - 2022.03135.PTDC.