Elucidating the role of SNARE Syntaxin 1A in regulating lysosome and melanosome function

Abstract

SNAREs are transmembrane proteins that localize to specific membranes and facilitate the process of membrane/vesicle fusion. SNAREs mediate the fusion by binding with their respective cognate SNAREs. SNAREs mediate the fusion by binding with their respective cognate SNAREs. The typical structure of a SNARE contains a SNARE motif which consists of typically 60-70 amino acids and interacts with other cognate SNAREs; the N-terminal regulatory; and the transmembrane domain (TM) at the C-terminus. Exceptionally, only some SNAREs may have a TM domain or an unstructured N-terminal region (Fasshauer, 2003). Further, SNAREs are classified into Q- and R- SNAREs. The Q-SNARE family comprises several members of syntaxins (STX), including STX1A and STX1B, which were extensively described in the literature. STX1A or 1B SNAREs are believed to function only in neuronal cells. However, further studies have shown that they are present in other cells as well (Bennett, Calakos, & Scheller, 1992) (Bennett et al., 1993). Studies have also shown that STX1A localizes to endosomes (Brandhorst et al., 2006). Proteomics analysis of a Drosophila S2 cell line revealed that only one isoform, STX1 is present in the cell line, and it localizes at phagosomes (Stuart et al., 2007). However, the precise function of endosomal STX1 or at phagosomes is not yet well-understood (Dingjan et al., 2018). Although STX1A is known to function majorly at the plasma membrane (PM), its intracellular localization or function remains poorly understood. In this study, we have attempted to study the intracellular function of STX1A using HeLa cells/melanocytes as a model. To elucidate the role of SNARE STX1A in regulating lysosome and melanosome function, we have divided the work into three objectives. We have studied the localization, its functional regulation in HeLa cells and targeting mechanisms of STX1A to the intracellular organelles (Objective 1), screening and identification of cognate SNAREs of STX1A (Objective 2), and its role in melanosome biogenesis (Objective 3). These studies demonstrate that STX1A regulates lysosome exocytosis and melanosome biogenesis in mammalian cells.