Apoptosome formation upon overexpression of native and truncated Apaf-1 in cell-free and cell-based systems

Apaf-1 is a cytosolic multi-domain protein in the apoptosis regulatory network. When cytochrome c releases from mitochondria; it binds to WD-40 repeats of Apaf-1 molecule and induces oligomerization of Apaf-1. Here in, a split luciferase assay was used to compare apoptosome formation in cell-free and cell-based systems. This assay uses Apaf-1 tagged with either N-terminal fragment or C-terminal fragment of P. pyralis luciferase. In cell based-system, the apoptosome formation is induced inside the cells which express Apaf-1 tagged with complementary fragments of luciferase while in cell-free system, the apoptosome formation is induced in extracts of the cells. In cell-free system, cytochrome c dependent luciferase activity was observed with full length Apaf-1. However, luciferase activity due to apoptosome formation was much higher in cell based system compared to cell-free system. The truncated Apaf-1 which lacks WD-40 repeats (ΔApaf-1) interacted with endogenous Apaf-1 in a different fashion compared to native form as confirmed by different retention time of eluate in gel filtration and binding to affinity column. The interactions between endogenous Apaf-1 and ΔApaf-1 is stronger than its interaction with native exogenous Apaf-1 as indicated by dominant negative effect of ΔApaf-1 on caspase-3 processing.