The interaction of PP1 with BRCA1 and analysis of their expression in breast tumors

<p>Abstract</p> <p>Background</p> <p>The breast cancer susceptibility gene, <it>BRCA1</it>, is implicated in multiple cellular processes including DNA repair, the transactivation of genes, and the ubiquitination of proteins; however its precise functions remain to be fully understood. Identification and characterization of BRCA1 protein interactions may help to further elucidate the function and regulation of BRCA1. Additionally, detection of changes in the expression levels of <it>BRCA1 </it>and its interacting proteins in primary human breast tumors may further illuminate their role in the development of breast cancer.</p> <p>Methods</p> <p>We performed a yeast two-hybrid study to identify proteins that interact with exon11 of BRCA1 and identified Protein Phosphatase 1β (PP1β), an isoform of the serine threonine phosphatase, PP1. GST-pull down and co-immunoprecipitation assays were performed to further characterize this interaction. Additionally, Real-Time PCR was utilized to determine the expression of <it>BRCA1</it>, <it>PP1</it>α, β and γ in primary human breast tumors and normal breast tissue to identify alterations in the expression of these genes in breast cancer.</p> <p>Results</p> <p>PP1 and BRCA1 co-immunoprecipitate and the region within BRCA1 as well as the specific PP1 interacting domain mediating this interaction were identified. Following mRNA expression analysis, we identified low levels of <it>BRCA1 </it>and variable levels of <it>PP1</it>α and β in primary sporadic human breast tumors. Furthermore, BRCA1, <it>PP1</it>β and PP1γ were significantly higher in normal tissue specimens (BRCA1 p = 0.01, <it>PP1</it>β: p = 0.03, <it>PP1</it>γ, p = 1.9 × 10^-6) compared to sporadic breast tumor samples. Interestingly, we also identified that ER negative tumors are associated with low levels of <it>PP1</it>α expression.</p> <p>Conclusion</p> <p>The identification and characterization of the interaction of BRCA1 with PP1 and detection of changes in the expression of <it>PP1 </it>and genes encoding other BRCA1 associated proteins identifies important genetic pathways that may be significant to breast tumorigenesis. Alterations in the expression of genes, particularly phosphatases that operate in association with BRCA1, could negatively affect the function of BRCA1 or BRCA1 associated proteins, contributing to the development of breast cancer.</p

Synaptotagmins I and IV promote transmitter release independently of Ca2+ binding in the C2A domain

At nerve terminals, a focal and transient increase in intracellular Ca2+ triggers the fusion of neurotransmitter-filled vesicles with the plasma membrane. The most extensively studied candidate for the Ca2+-sensing trigger is synaptotagmin I, whose Ca2+-dependent interactions with acidic phospholipids and syntaxin1 have largely been ascribed to its C2A domain, although the C2B domain also binds Ca2+ (refs 7, 8). Genetic tests of synaptotagmin I have been equivocal as to whether it is the Ca2+ -sensing trigger of fusion. Synaptotagmin IV, a related isoform that does not bind Ca2+ in the C2A domain, might be an inhibitor of release. We mutated an essential aspartate of the Ca2+-binding site of the synaptotagmin I C2A domain and expressed it in Drosophila lacking synaptotagmin I. Here we show that, despite the disruption of the binding site, the Ca2+-dependent properties of transmission were not altered. Similarly, we found that synaptotagmin IV could substitute for synaptotagmin I. We conclude that the C2A domain of synaptotagmin is not required for Ca2+-dependent synaptic transmission, and that synaptotagmin IV promotes rather than inhibits transmission