The yeast ARL1 gene, encoding a guanine-nucleotide binding protein of the Arf-like family, exhibits a synthetic genetic interaction with CCZ1. An arl1Delta ccz1Delta double mutant was viable but grew slowly, was more sensitive to caffeine, Ca2+, Zn2+, and hygromycin B than either single mutant, and had a more severe vacuolar protein sorting phenotype. Overexpression of ARL1 did not suppress ccz1Delta mutant phenotypes, nor did overexpression of CCZ1 suppress arl1Delta mutant phenotypes. We conclude that ARL1 and CCZ1 independently contribute to both ion homeostasis and protein sorting. (C) 2004 Elsevier Inc. All rights reserved.