Parafusin and calcium-induced signal transduction in exocytosis

The phosphoglycoprotein parafusin is a member of the phosphoglucomutase superfamily and has been shown, both via biochemical and localization studies, to be associated with the Ca²⁺dependent regulated exocytosis process in Paramecium tetraurelia. Stimulation of exocytosis in this cell leads to a Ca²⁺-dependent glucosylation of parafusin accompanied by its dissociation from the secretory vesicles and from cell membrane docking sites. These events are blocked in the presence of extracellular Mg²⁺ in wild-type cells and in either Ca²⁺ or Mg²⁺ in a temperature-sensitive mutant, nd9, stimulated at the nonpermissive temperature. Furthermore, laser scanning confocal localization studies with antibodies to parafusin whole protein versus antibody made to a specific peptide (insertion 2) show different localization patterns. While insertion-2 antibodies only label the organelles previously shown to have parafusin associated with them, i.e., cell membrane fusion (docking) sites and secretory vesicles, antibodies to whole protein outline in addition the alveolar sacs (subsurface cisterns) which are Ca²⁺ storage compartments in this cell. This may indicate tht other members of the phosphoglucomutase superfamily which interact specifically with this compartment are present in P. tetraurelia.