Rhomboids

Abstract

What are they? Rhomboids are a recently discovered family of serine proteases that cleave substrates within transmembrane domains. All rhomboids have multiple transmembrane domains (typically six or seven) and it is proposed that they work by forming the classical charge relaying catalytic triad – famous from serine proteases such as chymotrypsin – between their transmembrane domains. Hang on! Surely proteolysis can't work in the hydrophobic membrane bilayer…? Actually, the rhomboid family joins an exclusive club of proteases that appear to do just that. 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. It's probably no coincidence that, although these families are unrelated, they all have multiple transmembrane domains: this may produce a hydrophilic microenvironment around the active site of hydrolysis. So all this can actually be seen in crystal structures? No, it's inferred from biochemistry and genetics. Nobody has yet succeeded in solving the structure of any of these proteins. But to be fair, the evidence for the basic model is pretty strong. Why the weird name? Blame the Drosophilists. Rhomboid's first recognition came as a mutant discovered in the genetic screens that won Christiane Nüsslein-Volhard and Eric Wieschaus a Nobel Prize in 1995. Mutant Drosophila embryos have rhomboid-shaped heads. How were they discovered to be proteases? Genetics indicated that Rhomboid-1 in flies – we now know that Drosophila has seven rhomboids – is a key activator of epidermal growth factor (EGF) receptor signalling, and that it works in the signal-sending cell. Subsequent biochemistry and cell biology showed that Rhomboid-1 cleaves and thereby activates three membrane-tethered EGF-like ligands. This occurs in the Golgi apparatus, where many eukaryotic rhomboids reside. Do they all control EGF receptor signalling? No. For one thing, rhomboids exist throughout evolution – from bacteria and archaea to humans – and most organisms which have rhomboids don't even have receptor tyrosine kinases, let alone EGF receptors. But, amazingly, the bacterium Providencia stuartii uses a rhomboid to release an intercellular signalling molecule – a mechanism apparently analagous to the activation of EGF ligands by rhomboids in Drosophila – implying a common signalling mechanism between bacteria and metazoans. But they all control intercellular signalling of some kind? No again. Although we know rather little about the roles of rhomboids in most species, they probably do a wide variety of things – connected only by the basic mechanism of releasing a protein from a mem brane by proteolysis of its trans membrane domain. For example, eukaryotes all have a mitochondrial rhomboid that, at least in yeast, controls membrane fusion by cleaving a dynamin-like GTPase. Can rhomboid cleavage sites be recognised? Not very efficiently, though current evidence suggests that all rhomboids recognise, as well as cleave, transmembrane domain sequences. There is a requirement for residues that destabilise transmembrane α helices, and substrate transmembrane domains must also be relatively hydrophilic near their luminal end. These requirements have allowed some prediction of rhomboid substrates. What's the difference between rhomboids and other intramembrane proteases? Apart from the catalytic mechanism – rhomboids are the only intramembrane serine proteases – one important difference is that all known rhomboids release extracellular or luminal domains from transmembrane domains. 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.