Figure 4

VAPB interaction with FAF1 and p97 is stimulated upon proteasome inhibition. (A) Proteasome inhibition enhances ubiquitin, p97 and FAF1 binding to Flag-VAPB, while ASNA1 binding remains largely unchanged. U2OS cells expressing Flag-VAPB from a tetracycline-inducible promoter were grown in the presence of 100 ng/ml tetracycline for 24 hr or left untreated as a control. Flag-VAPB was immunoprecipitated using anti-Flag beads. (B) Proteasome inhibition enhances ubiquitin, p97 and FAF1 binding to endogenous VAPB, while ASNA1 binding was only slightly increased. Endogenous VAPB was immunoprecipitated from U2OS cells using anti-VAPB antibodies cross-linked to Protein A-beads. Uncoupled beads were used as control. (C, D) The binding of ubiquitinated proteins to VAPB is largely mediated by FAF1. U2OS cells were treated with the indicated siRNA oligos. Flag-VAPB (C) or endogenous VAPB (D) were immunoprecipitated as described above. Depletion of FAF1 strongly reduces the binding of ubiquitinated proteins to Flag- or endogenous VAPB. (E) The K87D M89D double mutant (KM-DD) of VAPB is defective in p97, FAF1, ASNA1 and ubiquitin binding. Flag-VAPB full-length WT, KM-DD and truncated, C-terminal half (C-Ter) and N-terminal half (MSP), were immunoprecipitated from U2OS cells. The MSP domain of VAPB is sufficient to interact with p97, FAF1 and ASNA1. KM-DD as well as the truncation lacking the MSP domain (C-Ter) are defective in binding poly-ubiquitinated proteins and seem to interact preferentially with oligo-ubiquitinated proteins. (F) The binding of ubiquitinated proteins to VAPB is reduced in cells treated with ASNA1 siRNA. U2OS cells expressing Flag-VAPB from a tetracycline-inducible promoter were treated with the indicated siRNA oligos. Depletion of ASNA1 reduces ubiquitin and BAG6 binding, but not FAF1 binding, to Flag-VAPB. C-Ter, C-terminal half; IP, immunoprecipitate; KM-DD, K87D M89D double mutant; Luc, luciferase.