Structurally conserved interaction of Lgl family with SNAREs is critical to their cellular function

The Lethal giant larvae (Lgl) tumor suppressor family is conserved from yeast to mammals and plays a critical yet controversial role in cell polarity. Studies on Drosophila Lgl suggest that its function in polarity is through regulation of the acto-myosin cytoskeleton. In contrast, studies on the yeast Lgl homologs, Sro7/ Sro77, suggest a function in exocytosis through interaction with the t-SNARE Sec9. Using yeast/mammalian Lg1 chimeras, we demonstrate that the overall architecture of Lg1 proteins is highly conserved and that the C-terminal domain is the major site of SNARE interaction within both yeast and mammalian homologs. Importantly, we find that the ability of Lg1 chimeras to function as the only source of Lgl in yeast correlates precisely with the ability to interact with the yeast t-SNARE. We report a novel interaction between Sro7 and the yeast myosin V, Myo2. However, we find that interactions with either Myo2 or Myo1 (myosin II) cannot account for the dramatic functional differences observed for these chimeras in yeast. These results provide the first demonstration that the interaction of an Lg1 family member with a specific effector is critical to its function in vivo. These data support the model that the Lg1 family functions in cell polarity, at least in part, by regulating SNARE-medlated membrane delivery events at the cell surface.