Multiple sites of action of Ccz1p - the Saccharomyces cerevisiae homolog of the human HPS4 protein
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The protein Ccz1p, from the yeast Saccharomyces cerevisiae, was known to function in homotypic vacuolar fusion events, as part of the Ccz1p-Mon1p complex that regulates the Rab-GTPase Ypt7p. The aim of this work was to investigate the possibility that Ccz1p performs also other functions or acts at other sites in the yeast cell. The study utilizes genetic, biochemical and cell biology methods, as well as protein sequence analysis which led to the identification of a conserved protein domain, termed CHiPS, that shows homology to a domain present in the human protein HPS4. Despite sequence similarity, the human CHiPS domain was found non-functional in yeast cells. It was established that both the N-terminal CHiPS domain and the C-terminal tail domain of Ccz1p are indispensable for its functioning, since truncation on the N- or C-terminus renders the protein completely inactive. Finally, a novel function was identified for the regulatory complex Ccz1p-Mon1p and for Ypt7p: they are necessary both for efficient inheritance of vacuoles and for correct functioning of the Kar9p-dependent nuclear inheritance pathway.