MYO18B interacts with the proteasomal subunit Sug1 and is degraded by the ubiquitin-proteasome pathway

MYO18B is a class XVIIIB unconventional myosin encoded by a candidate tumor suppressor gene. To gain insights into the cellular function of this protein, we searched for MYO18B-interacting proteins by a yeast two-hybrid screen. Sug1, a 19S regulator subunit of the 26S proteasome, was identified as a binding partner of the C-terminal tail region of MYO18B. The association of MYO18B with Sug1 was further confirmed by GST pull-down, co-immunoprecipitation, and immunocytochemistry. Furthermore, proteasome dysfunction by a proteasome inhibitor or siRNA-mediated knock-down of Sug1 caused the up-regulation of MYO18B protein and MYO18B was polyubiquitinated in vivo. Collectively, these results suggested that MYO18B is a substrate for proteasomal degradation.