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This Article Full Text Full Text (PDF) All Versions of this Article: 295/4/C975    most recent 90619.2007v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Qi, J. Articles by Banes, A. J. PubMed PubMed Citation Articles by Qi, J. Articles by Banes, A. J.

GROWTH, DIFFERENTIATION, AND APOPTOSIS

Nuclear localization of the titin Z1Z2Zr domain and role in regulating cell proliferation

¹Flexcell International Corporation, Hillsborough, North Carolina; ²Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Raleigh, North Carolina; and ³Institute for Anesthesiology and Intensive Operative Care, University Hospital, Mannheim, Germany

Submitted 10 December 2007 ; accepted in final form 4 August 2008

Titin (also called connectin) is a major protein in sarcomere assembly as well as providing elastic return of the sarcomere postcontraction in cardiac and striated skeletal muscle tissues. In addition, it has been speculated that titin is associated with nuclear functions, including chromosome and spindle formation, and regulation of muscle gene expression. In the present study, a short isoform of titin was detected in a human osteoblastic cell line, MG-63 cells, by both immunostaining and Western blot analysis. Confocal images of titin staining showed both cytoplasmic and nuclear localization in a punctate pattern. Therefore, we hypothesized that human titin may contain a nuclear localization signal (NLS). A functional NLS, 200-PAKKTKT-206, located in a low-complexity, titin-specific region between Z2 and Z repeats, was found by sequentially deleting segments of the NH₂-terminal sequence in conjunction with an enhanced green fluorescent protein reporter system and confirmed by site-directed mutagenesis. Overexpression of titin's amino terminal fragment (Z1Z2Zr) in human osteoblasts (MG-63) increased cell proliferation by activating the Wnt/β-catenin pathway. RT-PCR screens of tissue panels demonstrated that residues 1–206 were ubiquitously expressed at low levels in all tissues and cell types analyzed. Our data implicate a dual role for titin's amino terminal region, i.e., a novel nuclear function promoting cell division in addition to its known structural role in Z-line assembly.

Wnt; catenin; osteoblast