The Dictyostelium discoideum family of Rho-related proteins

Taking advantage of the ongoing Dictyostelium genome sequencing project, we have assembled >73kb of genomic DNA in 15 contigs harbouring 15 genes and one pseudogene of Rho-related proteins, Comparison with EST sequences revea led that every gene is interrupted by at least one and up to four introns. For racC extensive alternative splicing was identified. Northern blot analy sis showed that mRNAs for racA, racE, racG, racH and racl were present at a ll stages of development, whereas racJ and racL were expressed only at late stages, Amino acid sequences have been analysed in the context of Rho-rela ted proteins of other organisms, Rac1a/1b/1c, RacF1/F2 and to a lesser exte nt RacB and the GTPase domain of RacA can be grouped in the Rac subfamily. None of the additional Dictyostelium Rho-related proteins belongs to any of the well-defined subfamilies, like Rac, Cdc42 or Rho, RacD and RacA are un ique in that they lack the prenylation motif characteristic of Rho proteins . RacD: possesses a 50 residue C-terminal extension and RacA a: 400 residue C-terminal extension that contains: a proline-rich region, two BTB domains and a novel C-terminal domain. We have also identified homologues: for Rac A in Drosophila and mammals, thus defining a new subfamily of Rho proteins, RhoBTB.