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Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, Illinois 60637
Phosphoglucomutase (PGM) is a ubiquitous
highly conserved enzyme involved in carbohydrate metabolism. A number of recently discovered PGM-like
proteins in a variety of organisms have been proposed
to function in processes other than metabolism. In addition, sequence analysis suggests that several of these
may lack PGM enzymatic activity. The best studied
PGM-like protein is parafusin, a major phosphoprotein
in the ciliate Paramecium tetraurelia that undergoes
rapid and massive dephosphorylation when cells undergo synchronous exocytosis of their dense-core secretory granules. Indirect genetic and biochemical evidence also supports a role in regulated exocytotic
membrane fusion. To examine this matter directly, we
have identified and cloned the parafusin homologue in
Tetrahymena thermophila, a ciliate in which protein
function can be studied in vivo. The unique T. thermophila gene, called PGM1, encodes a protein that is
closely related to parafusin by sequence and by characteristic post-translational modifications. Comparison of
deduced protein sequences, taking advantage of the known atomic structure of rabbit muscle PGM, suggests that both ciliate enzymes and all other PGM-like
proteins have PGM activity. We evaluated the activity
and function of PGM1 through gene disruption. Surprisingly, 
PGM1 cells displayed no detectable defect
in exocytosis, but showed a dramatic decrease in PGM
activity. Both our results, and reinterpretation of previous data, suggest that any potential role for PGM-like
proteins in regulated exocytosis is unlikely to precede
membrane fusion.
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