FIGURE 7:

Model of the final stages of the tubulin-folding pathway. The dual functions of Tbc1 are shown in two parallel pathways. Top pathway shows the last steps of the tubulin-folding pathway as shown in Figure 1A after Tbc1 forms a supercomplex together with Alp1^D and Alp21^E. Lower pathway shows the alternative function of Tbc1 as a GAP for Alp41. The subsequently GDP-bound Alp41 is now able to bind to Alp1^D, thus preventing excess Alp1^D from localizing on the microtubules, where they would otherwise cause dissociation of microtubules. The lack of interaction of Alp41 and Alp1^D strongly suggests that the two arms of the pathway are independent of one another.