Component of the ESCRT-III complex, which is required for multivesicular bodies (MVBs) formation and sorting of endosomal cargo proteins into MVBs. The MVB pathway mediates delivery of transmembrane proteins into the lumen of the lysosome for degradation. The ESCRT-III complex is probably involved in the concentration of MVB cargo. May also be involved in chromosome condensation. Targets the Polycomb group (PcG) protein PCGF4/BMI1 to regions of condensed chromatin. May play a role in stable cell cycle progression and in PcG gene silencing.