The carboxy-terminal hydrophobic domain of TIG3, a class II tumor suppressor protein, is required for appropriate cellular localization and optimal biological activity

TIG3 is a recently discovered class II tumor suppressor protein, originally isolated from retinoid-treated cultured epidermal keratinocytes, that supp resses the proliferation of a variety of epithelial cell types. In the pres ent study, we examine the ability of this protein to reduce CHO, T47D and H aCaT cell proliferation, and the role of the carboxy-terminal hydrophobic d omain in this regulation. Vector-mediated expression of the full length TIG 3 protein, TIG3(1-164), results in a 50-70% reduction colony formation effi ciency. Expression of a truncated mutant, TIG3(1-134), that lacks the putat ive carboxy-terminal membrane-anchoring domain, results in a partial loss o f ability to suppress colony formation. The fact that the truncated protein remains partially active suggests that both the amino- and cal carboxy-ter minal regions of TIG3 are required fur optimal growth suppression. The full -length protein is distributed in a perinuclear location, and is not presen t in the nucleus. TIG3(1-134), in contrast, is distributed in the cytoplasm . Thus, a change in location is associated with the partial loss of activit y. We also monitored the distribution of green fluorescent protein (GFP)-TI G3 fusion proteins. GFP-TIG3(1-164) was localized in a pattern similar to t hat observed for TIG3(1-164), while GFP-TIG3(1-134) displayed a distributio n pattern similar to GFP. This suggests that the C-terminal hydrophobic dom ain has an important role in determining the intracellular localization of TIG3. In addition, GFP-TIG3(1-164) retains the ability to inhibit cell func tion, while GFP-TIG3(1-134) is inactive.