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PubMed2001

Organization of the Rab-GDI/CHM superfamily: the functional basis for choroideremia disease.

Choroideremia is an X-chromosome-linked disease that leads to the degeneration of the choriocapillaris, the retinal pigment epithelium and the photoreceptor layer in the eye. The gene product defective in choroideremia, CHM, is identical to Rab escort protein 1 (REP1). CHM/REP1 is an essential component of the catalytic geranylgeranyltransferase II complex (GGTrII) that delivers newly synthesized small GTPases belonging to the RAB gene family to the catalytic complex for post-translational modification. CHM/REP family members are evolutionarily related to members of the guanine nucleotide dissociation inhibitor (GDI) family, proteins involved in the recycling of Rab proteins required for vesicular membrane trafficking through the exocytic and endocytic pathways, forming the GDI/CHM superfamily. Biochemical and structural analyses have now revealed a striking parallel in the organization and function of these two families allowing us to generate a general model for GDI/CHM superfamily function in health and disease.

Adaptor Proteins, Signal Transducing↗

PubMed2000

Molecular basis for Rab prenylation.

Rab escort proteins (REP) 1 and 2 are closely related mammalian proteins required for prenylation of newly synthesized Rab GTPases by the cytosolic heterodimeric Rab geranylgeranyl transferase II complex (RabGG transferase). REP1 in mammalian cells is the product of the choroideremia gene (CHM). CHM/REP1 deficiency in inherited disease leads to degeneration of retinal pigmented epithelium and loss of vision. We now show that amino acid residues required for Rab recognition are critical for function of the yeast REP homologue Mrs6p, an essential protein that shows 50% homology to mammalian REPs. Mutant Mrs6p unable to bind Rabs failed to complement growth of a mrs6Delta null strain and were found to be dominant inhibitors of growth in a wild-type MRS6 strain. Mutants were identified that did not affect Rab binding, yet prevented prenylation in vitro and failed to support growth of the mrs6Delta null strain. These results suggest that in the absence of Rab binding, REP interaction with RabGG transferase is maintained through Rab-independent binding sites, providing a molecular explanation for the kinetic properties of Rab prenylation in vitro. Analysis of the effects of thermoreversible temperature-sensitive (mrs6(ts)) mutants on vesicular traffic in vivo showed prenylation activity is only transiently required to maintain normal growth, a result promising for therapeutic approaches to disease.

Adaptor Proteins, Signal Transducing↗