Human Vam6p promotes lysosome clustering and fusion in vivo

Regulated fusion of mammalian lysosomes is critical to their ability to acq uire both internalized and biosynthetic materials. Here, we report the iden tification of a novel human protein, hVam6p, that promotes lysosome cluster ing and fusion in vivo. Although hVam6p exhibits homology to the Saccharomy ces cerevisiae vacuolar protein sorting gene product Vam6p/Vps39p, the pres ence of a citron homology (CNH) domain at the NH2 terminus is unique to the human protein. Overexpression of hVam6p results in massive clustering and fusion of lysosomes and late endosomes into large (2-3 mum) juxtanuclear st ructures. This effect is reminiscent of that caused by expression of a cons titutively activated Rab7. However, hVam6p exerts its effect even in the pr esence of a dominant-negative Rab7, suggesting that it functions either dow nstream of, or in parallel to, Rab7. Data from gradient fractionation, two- hybrid, and coimmunoprecipitation analyses suggest that hVam6p is a homooli gomer and that its self-assembly is mediated by a clathrin heavy chain repe at domain in the middle of the protein. Both the CNH and clathrin heavy cha in repeat domains are required for induction of lysosome clustering and fus ion. This study implicates hVam6p as a mammalian tethering/docking factor c haracterized with intrinsic ability to promote lysosome fusion in vivo.