Rhomboids - unusual proteases with many biological functions

We discovered that rhomboids are a novel family of serine proteases that cleave substrates within transmembrane domains (TMDs). All rhomboids have multiple TMDs (typically six or seven). Their mechanism depends on an unusual catalytic dyad between conserved serine and histidine residues positioned in hydrophobic TMD-segments. The crystal structures of rhomboids provide significant clues about their detailed mechanism.

The rhomboid family joins an exclusive club of proteases that have the unusual property of cleaving proteins within their TMDs – actually cutting in the membrane bilayer. The presenilins and signal peptide peptidase are intramembrane aspartyl proteases; they cleave substrates including the Notch receptor and the amyloid precursor protein, which is implicated in Alzheimer's disease. The site-2 protease family are intramembrane metalloproteases; among other things, they regulate cholesterol biosynthesis. The different intramembrane protease families are evolutionarily and mechanistically unrelated, but there are clear common functional themes that link them.

Although we discovered rhomboid's function as an activator of EGF receptor signalling in Drosophila, they exist throughout evolution – from bacteria and archaea to humans – so the majority of organisms which have rhomboids don't have receptor tyrosine kinases, let alone EGF receptors. We are currently trying to discover what their role is in a variety of organisms. Our current evidence suggests that Rhomboids participate in much more than just intercellular signalling. For example, eukaryotes all have a mitochondrial rhomboid that, at least in yeast, controls membrane fusion by cleaving a dynamin-like GTPase.

Apart from their catalytic mechanisms (rhomboids are the only intramembrane serine proteases), one important difference between rhomboids and all other intramembrane proteases is that all known rhomboids release extracellular or luminal domains from TMDs. The others all release cytoplasmic domains, usually tethered transcription factors. By allowing intramembrane proteolysis to occur in the other direction, rhomboids therefore double the potential of intramembrane proteolysis to be a widespread signalling mechanism.

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