The Kelch protein NS1-BP interacts with alpha-enolase/MBP-1 and is involved in c-Myc gene transcriptional control.

Alpha-enolase is a key glycolytic enzyme that plays a functional role in several physiological processes depending on the cellular localization. The enzyme is mainly localized in the cytoplasm whereas an alternative translated form, named MBP-1, is predominantly nuclear. The MBP-1 protein has been characterized as a c-Myc promoter binding protein that negatively controls transcription. In the present study, we identified the kelch protein NS1-BP as one of the alpha-enolase/MBP-1 partners by using a yeast two-hybrid screening. Although NS1-BP has been originally described as a protein mainly localized in the nucleus, we provide evidence that NS1-BP also interacts with actin in human cells, as reported for most kelch-containing proteins. Here we showed that alpha-enolase and MBP-1 associate with NS1-BP in vitro and in vivo by GST pull-down assays and coimmunoprecipitation experiments; subsequent immunofluorescent staining confirmed colocalization of the proteins within the cells. Furthermore, functional analyses performed by cotransfection assays revealed that NS1-BP enhances the inhibitory effect exerted by MBP-1 on c-Myc promoter. In mammalian cells, the overexpression of both proteins resulted in an increased repression of basal c-Myc transcription and consistently affected the steady state levels of endogenous c-Myc mRNA. These findings further support the distinct roles of alpha-enolase and its MBP-1 variant in maintaining cell homeostasis. Moreover, our data suggest a novel function for NS1-BP in the control of cell proliferation.