TSG101 protein steady-state level is regulated posttranslationally by an evolutionarily conserved COOH-terminal sequence.

Antisense inactivation of the tsg101 tumor susceptibility gene in murine NIH3T3 fibroblasts leads to neoplastic transformation and tumorigenesis, which are reversed by restoration of tsg101 activity. tsg101 deficiency is associated with a series of mitosis-related abnormalities, whereas overexpression of TSG101 can also result in neoplastic transformation and the perturbation of cell cycling. Together, these observations imply that TSG101 production outside of a narrow range can lead to abnormal cell growth. We report here that the TSG101 protein is maintained at an almost constant steady-state level in cultured murine and human cells and that this occurs through a posttranslational process involving TSG101 protein degradation. Sustained overproduction of TSG101 from chromosomally inserted adventitious constructs resulted in compensatory down-regulation of endogenous TSG101 and replacement of the native protein by the adventitious one. Using deletion mutants of TSG101, we mapped the region responsible for autoregulation of the TSG101 steady-state level to an evolutionarily conserved sequence, here termed the "steadiness box," located near TSG101's COOH-terminal end. Our results suggest a model in which the biological effects of TSG101 are modulated either by self-promoted proteolysis or participation with other cellular protein(s) in a proteolytic feedback-control loop.