Nonphototrophic hypocotyl 3 domain proteins: traffic directors, hitchhikers, or both?

SummaryThe nonphototrophic hypocotyl 3 (NPH3) domain is plant specific and of unknown function. It is nearly always attached to an N‐terminal BTB domain and a largely unstructured C‐terminal region. Recent reports revealed NPH3‐domain GTPase activity and connection to intracellular trafficking, condensate formation, membrane attachment of the C‐terminal region for some NPH3‐domain proteins and, at the physiological level, drought‐related function for at least one NPH3‐domain protein. We integrate these new ideas of NPH3‐domain protein function into two, nonexclusive, working models: the ‘traffic director’ model, whereby NPH3‐domain proteins regulate intracellular trafficking and, the ‘hitchhiker’ model whereby NPH3‐domain proteins ride the trafficking system to find ubiquitination targets. Determining which model best applies to uncharacterized NPH3‐domain proteins will contribute to understanding intracellular trafficking and environmental responses.