Thrifty Tbc1d1 and Tbc1d4 proteins link signalling and membrane trafficking pathways.

Establishing a complete pathway which links occupancy of the insulin receptor to GLUT4 translocation has been particularly elusive because of the complexities involved in studying both signalling and membrane trafficking processes. However, Lienhard's group has now discovered two related molecules that could function in this linking role. These proteins, Tbc1d4 (also known as AS160) and now Tbc1d1, as reported in this issue of the Biochemical Journal, have been demonstrated to be Rab GAPs (GTPase-activating proteins) that link upstream to Akt (protein kinase B) and phosphoinositide 3-kinase and downstream to Rabs involved in trafficking of GLUT4 vesicles. The data from Leinhard and colleagues suggest that high levels of Rab GAP activity lead to suppression of GLUT4 translocation and this observation has wide significance and is likely to be relevant to the recent discovery that mutations in the Tbc1d1 gene lead to some cases of severe human obesity.