Endocrine Abstracts

Parafibromin is a tumour suppressor protein, encoded by the gene responsible for the hyperparathyroidism-jaw tumour (HPT-JT) syndrome, which is an autosomal dominant disorder, characterised by parathyroid tumours, ossifyfing fibromas, renal and uterine tumours. Parafibromin is a component of the polymerase-associated factor 1 (PAF1) complex and interacts directly with β-catenin, although these functions do not fully explain its role in tumourigenesis. To further investigate these roles of parafibromin, we embarked on identifying its interacting proteins by a yeast two-hybrid screen using a full-length human parafibromin cDNA construct and a renal cDNA library. This identified two proteins, transforming acidic coiled coil family member 3 (TACC3) and kinesin family member 5B (KIF5B). The potential interactions, between parafibromin and TACC3 and KIF5B, were assessed in mammalian cells by endogenous coimmunoprecipitation using human embryonic kidney (HEK293) cell lysates and antibodies to parafibromin, TACC3 and KIF5B. The interactions of parafibromin with the human PAF1 complex member hLeo1, and β-catenin in lithium chloride stimulated cells were used as positive controls. Parafibromin was found to coimmunoprecipitate KIF5B and TACC3, thereby indicating that it directly interacted in mammalian cells with these proteins that are involved in microtubule-based functions and in the case of TACC3, mitotic chromosome alignment. The functional significance of these putative interactions during mitosis was assessed by decreasing parafibromin expression by >95%, using a siRNA-parafibromin construct in HeLa cells, whose cell-cycle was synchronised by a double thymidine block. Immunocytochemistry using antibodies against α-tubulin, the centrosome marker pericentrin and mitotic marker phosphohistone-H3 did not detect any mitotic or spindle abnormalities. Thus, these results indicate that parafibromin interacts with TACC3 and KIF5B in mammalian cells and that this interaction may have a functional role in cellular processes other than mitosis. Identification of these functions may help to further elucidate the role of parafibromin in tumourigenesis.