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Vol. 17, Issue 3, 1388-1398, March 2006

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Identification of the Junctional Plaque Protein Plakophilin 3 in Cytoplasmic Particles Containing RNA-binding Proteins and the Recruitment of Plakophilins 1 and 3 to Stress Granules

Ilse Hofmann ^*, Marialuisa Casella , Martina Schnölzer , Tanja Schlechter ^*, Herbert Spring , and Werner W. Franke ^*

^* Division of Cell Biology, German Cancer Research Center, D-69120 Heidelberg, Germany; Protein Analysis Facility, German Cancer Research Center, D-69120 Heidelberg, Germany; and Biomedical Structure Analysis Group, German Cancer Research Center, D-69120 Heidelberg, Germany

Submitted August 2, 2005; Revised December 20, 2005; Accepted December 28, 2005
Monitoring Editor: Asma Nusrat

Recent studies on the subcellular distribution of cytoplasmic plaque proteins of intercellular junctions have revealed that a number of such proteins can also occur in the cyto- and the nucleoplasm. This occurrence in different, and distant locations suggest that some plaque proteins play roles in cytoplasmic and nuclear processes in addition to their involvement in cell–cell adhesive interactions. Plakophilin (PKP) 3, a member of the arm-repeat family of proteins, occurs, in a diversity of cell types, both as an architectural component in plaques of desmosomes and dispersed in cytoplasmic particles. In immuno-selection experiments using PKP3-specific antibodies, we have identified by mass spectrometric analysis the following RNA-binding proteins: Poly (A) binding protein (PABPC1), fragile-X-related protein (FXR1), and ras-GAP-SH3-binding protein (G3BP). Moreover, the RNA-binding proteins codistributed after sucrose gradient centrifugation in PKP3-containing fractions corresponding to 25–35 S and 45–55 S. When cells are exposed to environmental stress (e.g., heat shock or oxidative stress) proteins FXR1, G3BP, and PABPC1 are found, together with PKP3 or PKP1, in "stress granules" known to accumulate stalled translation initiation complexes. Moreover, the protein eIF-4E and the ribosomal protein S6 are also detected in PKP3 particles. Our results show that cytoplasmic PKP3 is constitutively associated with RNA-binding proteins and indicate an involvement in processes of translation and RNA metabolism.

Abbreviations used: FXR1, fragile-X-related protein; G3BP, Ras-GAP-SH3-binding protein; PABPC1, cytoplasmic poly (A) binding protein; PKP, plakophilin.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Ilse Hofmann (i.hofmann{at}dkfz.de).

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