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

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.