Proline isomerization could be a potential regulatory mechanism of Rhomboid proteases

Rhomboid proteins belong to a family of intramembrane proteases that have the unique ability to cleave peptide bonds within the lipid bilayer. The mechanism by which they overcome the hydrophobicity of the lipid bilayer to carry out water‐requiring peptide cleavage remains largely unknown. Recent structural studies on an E. coli rhomboid have revealed a possible regulatory mechanism for rhomboid intramembrane proteolysis. L5, the loop between transmembrane helices 5 and 6 is thought to alternate between an open and closed conformation. Since L5 is positioned directly above the rhomboid active site, it is believed that the opening and closing of L5 regulates rhomboid activity. Using Saccharomyces cerevisiae r hom b oi d ‐2 (Rbd2) as bait, a membrane yeast two‐hybrid screen identified a proline isomerase, Cpr8 as a potential interacting protein. Proline isomerization has recently been shown to open the pore of a neurotransmitter‐gated ion channel. Like rhomboids, proline isomerases are highly conserved and are present in the secretory pathway and in mitochondria. Multiple sequence alignment indicates a conserved proline within the rhomboid L5 loop. We propose that proline isomerization within the rhomboid L5 loop alters its conformation and regulates rhomboid enzymatic activity.