Evidence for a role of transmembrane protein p25 in localization of protein tyrosine phosphatase TC48 to the ER

Abstract

T-cell protein tyrosine phosphatase gives rise to two splice isoforms: TC48, which is localized to the endoplasmic reticulum (ER) and TC45, a nuclear protein. The present study was undertaken to identify proteins that are involved in targeting TC48 to the ER. We identified two TC48-interacting proteins, p²⁵ and p²³, from a yeast two-hybrid screen. p²³ and p²⁵ are members of a family of putative cargo receptors that are important for vesicular trafficking between Golgi complex and ER. Both p²³ and p²⁵ associate with overexpressed TC48 in Cos-1 cells as determined by coimmunoprecipitation. A significant amount of TC48 colocalized initially with ERGIC and Golgi complex markers (in addition to ER and nuclear membrane localization) and was then retrieved to the ER. Coexpression with p²⁵ enhanced ER localization of TC48, whereas coexpression with p²³ resulted in its trapping in membranous structures. Coexpression of a p²⁵ mutant lacking the ER-localization signal KKxx resulted in enhanced Golgi localization of TC48. Forty C-terminal amino acid residues of TC48 (position 376-415) were sufficient for interaction with p²³ (but not with p²⁵) and targeted green fluorescence protein (GFP) to the Golgi complex. Targeting of GFP to the ER required 66 C-terminal amino acid residues of TC48 (position 350-415), which showed interaction with p²⁵ and p²³. We suggest that TC48 translocates to the Golgi complex along the secretory pathway, whereas its ER localization is maintained by selective retrieval enabled by interactions with p²⁵ and p²³.