The parafibromin tumor suppressor protein interacts with actin-binding proteins actinin-2 and actinin-3

Fourteen parafibromin interaction positive preys representing 10 independent clones encoding actinin-2 were isolated. Parafibromin interacted with muscle alpha-actinins (actinin-2 and actinin-3), but not with non-muscle alpha-actinins (actinin-1 and actinin-4). The parafibromin-actinin interaction was verified by yeast two-hybrid, GST pull-down, and co-immunoprecipitation. Yeast two-hybrid analysis revealed that the N-terminal region of parafibromin interacted with actinins. In actin sedimentation assays parafibromin did not dissociate skeletal muscle actinins from actin filaments, but interestingly, parafibromin could also bundle/cross-link actin filaments. Parafibromin was predominantly nuclear in undifferentiated proliferating myoblasts (C2C12 cells), but in differentiated C2C12 myotubes parafibromin co-localized with actinins in the cytoplasmic compartment.These data support a possible contribution of parafibromin outside the nucleus through its interaction with actinins and actin bundling/cross-linking. These data also suggest that actinins (and actin) participate in sequestering parafibromin in the cytoplasmic compartment.Parafibromin is encoded by the HRPT2 gene located on chromosome 1q31[1] (OMIM# 145001 and 607393). HRPT2 inactivating mutations in the germline occur in the hyperparathyroidism-jaw tumor (HPT-JT) syndrome, characterized by hyperparathyroidism (parathyroid adenoma or carcinoma) (90%), fibrous-ossifying jaw tumors (30%), and bilateral renal cysts (10%) [1,2]. Germline HRPT2 mutations have also been detected in some cases of familial hyperparathyroidism [1,3,4], and biallelic somatic inactivation of HRPT2 is common in parathyroid tumor of HPT-JT and in sporadic parathyroid cancer [5-7].Parafibromin has three possible nuclear localization signals; it is predominantly found in the nucleus [8-16] and in the nucleolus [17]. The C-terminal part of parafibromin (ORF = 531 amino acids) is homologous to yeast Cdc73p (ORF = 393 amino acids), an RNA polym