The CCZ1 (YBR131w) gene encodes a protein required for fusion of various transport intermediates with the vacuole. Ccz1p, in a complex with Mon1p, is a close partner of Ypt7p in the processes of fusion of endosomes to vacuoles and homotypic vacuole fusion. In this work, we exploited the Ca2+-sensitivity of the ccz1Delta Mutant to identify genes specifically interacting with CCZ1, basing on functional multicopy Suppression of calcium toxicity. The presented results indicate that Ccz1p functions in the cell either in association with Mon1p and Ypt7p in fusion at the vacuolar membrane, or-separately-with Arl1p at early steps of vacuolar transport. We also show that Suppression of calcium toxicity by the calcium pumps Pmr1p and Pmc1p is restricted only to the subset of mutants defective in vacuole morphology. The mechanisms of Ca2+-pump-mediated suppression also differ from each other, since the action of Pmr1p, but not Pmc1p, appears to require Arl1p function. (C) 2005 Elsevier Inc. All rights reserved.