Identifying membrane-bound transcriptional regulatory proteins from rare but evolutionarily conserved domain combinations

Transcriptional regulatory proteins, including transcription factors (TFs) and chromatin modifiers, perform their crucial cellular functions inside cell nuclei. However, unlike most TFs, membrane-bound transcription factors (MBTFs) are first anchored in membranes before nuclear translocation. Known MBTFs are vital for processes from myelin expression (MYRF) to cholesterol homeostasis (SREBP), yet their overall diversity remains uncharted. We hypothesized that additional membrane-bound transcriptional regulators (MBTRs) might exist and developed a bioinformatics screen that prioritizes membrane proteins likely to regulate transcription. Our approach leverages domain composition, assuming that surprising domain combinations suggest novel biological functions. We searched for rare but evolutionarily conserved combinations of transmembrane domains paired with domains likely involved in transcriptional regulation. Our method rediscovered known MBTFs and membrane-bound histone kinases, and identified novel MBTR candidates, including histone N-acetyltransferases and the prolactin regulatory element-binding protein PREB. Experimental validation indicates that PREB may undergo cleavage. Without its transmembrane domain, PREB exhibits nuclear localization and chromatin association in mouse embryonic stem cells. Furthermore, we identified a putative class of new MBTFs that share MYRF's DNA-binding domain. These findings underscore the utility of our method and provide a framework for investigating MBTRs that likely facilitate the integration of extracellular signals with transcriptional responses.