Transcriptional regulator that acts in the TORC1 signaling pathway to regulate energy homeostasis and promote survival during nutrient deprivation. Interacts with REPTOR to form a transcriptional activator complex that functions downstream of TORC1 to up-regulate the expression of most target genes induced by TORC1 inhibition. In the complex, acts to enhance the binding of the transcriptional activator REPTOR to the regulatory sequences of target genes. Under normal conditions TORC1 is active, inhibiting the formation of the REPTOR/REPTOR-BP complex by phosphorylating REPTOR and mediates its cytoplasmic retention by forming a docking site for 14-3-3 proteins. Upon TORC1 inhibition resulting from nutrient stress, REPTOR is recruited into the nucleus where it interacts with REPTOR-BP and together they maintain organismal metabolism by activating the expression of target stress response genes including those involved in glycogenesis and triglyceride biosynthesis. The complex also appears to negatively regulate some aspects of TORC1-dependent larval growth. Homodimer. Interacts (via C-terminus) with REPTOR (via C-terminus). Binds chromatin. Sensitive to nutrient stress. Larvae display no obvious phenotype under normal feeding conditions; larval growth and development is normal, and there is no effect on triglyceride and glycogen levels. However when mutants pupate and become adults they display reduced triglyceride and glycogen stores leading to adults dying within 18 hours of starvation whereas controls survive 2.5 days without food. Larvae are also sensitive to nutrient stress displaying 50% lethality when fed a low nutrient diet. Belongs to the bZIP family. ATF subfamily.