The tripartite architecture of the eukaryotic integral membrane protein zinc metalloprotease Ste24.

Abstract

Ste24 enzymes, a family of eukaryotic integral membrane proteins, are zinc metalloproteases (ZMPs) originally characterized as “CAAX proteases” targeting prenylated substrates, including a‐factor mating pheromone in yeast and prelamin A in humans. Recently, Ste24 was shown to also cleave nonprenylated substrates. Reduced activity of the human ortholog, HsSte24, is linked to multiple disease states (laminopathies), including progerias and lipid disorders. Ste24 possesses a unique “α‐barrel” structure consisting of seven transmembrane (TM) α‐helices encircling a large intramembranous cavity (~14 000 Å3). The catalytic zinc, coordinated via a HExxH…E/H motif characteristic of gluzincin ZMPs, is positioned at one of the cavity's bases. The interrelationship between Ste24 as a gluzincin, a long‐studied class of soluble ZMPs, and as a novel cavity‐containing integral membrane protein protease has been minimally explored to date. Informed by homology to well‐characterized soluble, gluzincin ZMPs, we develop a model of Ste24 that provides a conceptual framework for this enzyme family, suitable for development and interpretation of structure/function studies. The model consists of an interfacial, zinc‐containing “ZMP Core” module surrounded by a “ZMP Accessory” module, both capped by a TM helical “α‐barrel” module of as yet unknown function. Multiple sequence alignment of 58 Ste24 orthologs revealed 38 absolutely conserved residues, apportioned unequally among the ZMP Core (18), ZMP Accessory (13), and α‐barrel (7) modules. This Tripartite Architecture representation of Ste24 provides a unified image of this enzyme family.