Providencia may help find a function for a novel, widespread protein family

The seven transmembrane domain protein Rhomboid (RHO) is a key component of EGF-receptor signalling in Drosophila. Genetic evidence indicates it is needed in the activation of the TGFα-like ligand of the Drosophila EGFR (DER), Spitz (SPI). Most of the pathway components, including DER and a transmembrane, inactive form of Spitz, have a broad distribution. The limiting step for activation of the pathway is thought to be Spitz proteolytic release (reviewed in [1]). Like mammalian TGFα, SPI is likely to be cleaved by an ADAM-family metalloprotease, but no evidence has been obtained for this in flies. Instead, Star (S), a single-pass transmembrane protein, and RHO have been implicated in SPI activation. The mechanism underlying SPI activation by RHO and S is not known, but neither is directly required for SPI proteolytic cleavage [2]. The puzzling observation that removal of rho (now rho-1) has little or no effect on eye development [3], despite a strong requirement for active SPI, was recently resolved when a new member of the rho family was found which is expressed specifically in the eye. One of the first fly mutations affecting eye development to be isolated in the laboratory, rhoughoid, was found to uncover a new rhomboid, named rhomboid-3, that accounts for RHO function during eye development [4]. Additional rho-like sequences were also reported in the Drosophila genome [4], in C. elegans, rat and human [5]. Moreover an entire protein family comprising 47 entries has been assembled at the InterPro database (Integrated resource of protein domains and functional sites, accession number IPR002610). Entries include archaea, Gram-negative and Gram-positive bacteria, the two major yeasts (2 entries each), Arabidopsis (5), sugarcane (1), Caenorhabditis elegans (4), Drosophila (8), rat (1) and human (1). Most of the identified sequences predict multi-transmembrane domain proteins. The conserved region spans the transmembrane domains, where a stretch corresponding to domains 2–3 in Rho-1,2 and 3, respectively, contains three strongly conserved histidyl and one asparagyl residue in combination with a GASG motif. Figure 1a,b shows the most conserved residues within the RHO domain and a phylogenetic tree based on the alignment of this region for representative entries. In contrast, the amino terminus is highly divergent already within the Drosophila Rhomboid group. The fact that RHO-like proteins are represented in such diverse phyla implies a more general cellular role than EGFR activation. The conservation of key amino acids within the transmembrane domains could correspond to a catalytic …