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 suppresses the proliferation of a variety of epithelial cell types. In the present study, we examine the ability of this protein to reduce CHO, T47D and HaCaT cell proliferation, and the role of the carboxy-terminal hydrophobic domain in this regulation. Vector-mediated expression of the full length TIG3 protein, TIG31-164, results in a 50-70% reduction colony formation efficiency. Expression of a truncated mutant, TIG31-134, that lacks the putative carboxy-terminal membrane-anchoring domain, results in a partial loss of ability to suppress colony formation. The fact that the truncated protein remains partially active suggests that both the amino- and carboxy-terminal regions of TIG3 are required for optimal growth suppression. The full-length protein is distributed in a perinuclear location, and is not present in the nucleus. TIG31-134, in contrast, is distributed in the cytoplasm. Thus, a change in location is associated with the partial loss of activity. We also monitored the distribution of green fluorescent protein (GFP)-TIG3 fusion proteins. GFP-TIG31-164 was localized in a pattern similar to that observed for TIG31-164, while GFP-TIG31-134 displayed a distribution pattern similar to GFP. This suggests that the c-terminal hydrophobic domain has an important role in determining the intracellular localization of TIG3. In addition, GFP-TIG31-164 retains the ability to inhibit cell function, while GFP-TIG31-134 is inactive.