Comparison of Dictyostelium LvsB and endosomal fission defect mutants support a fusion regulatory role for LvsB

Abstract

Defects in human Lyst are associated with the lysosomal disorder Chediak Higashi Syndrome. The absence of Lyst results in the formation of enlarged lysosome related compartments but the mechanism for how these compartments arise is not well established. Two opposing models have been proposed to explain Lyst function. The fission model describes Lyst as a positive regulator of fission from lysosomal compartments, while the fusion model identifies Lyst as a negative regulator of fusion between lysosomal vesicles. Here we used assays that can distinguish between defects in vesicle fusion versus fission. We compared the phenotype of Dictyostelium cells defective in LvsB, the ortholog of Lyst, with that of two known fission defect mutants (μ3 and WASH null mutants). We found that the temporal localization characteristics of the post-lysosomal marker vacuolin, as well as vesicular acidity and fusion dynamics of LvsB null cells are distinct from those of both μ3 and WASH null fission defect mutants. These distinctions are predicted by the fusion defect model and implicate LvsB as a negative regulator of vesicle fusion.