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In Saccharomyces cerevisiae, the class C Vps proteins, together with Vam2p/Vps41p and Vam6p/Vps39p form a complex that interacts with SNARE and Rab proteins to" tether" vacuolar membranes prior to fusion. To determine a role for the corresponding mammalian orthologues, we examined the function, localization, and protein interactions of endogenous mVps11, mVps16, mVps18, mVam2p and mVam6. We found a significant proportion of these proteins localized to EEA1 and Transferrin Receptor positive early endosomes in Vero, NRK and CHO cells. Immunoprecipitation experiments showed that mVps18 not only interacted with Syntaxin (Syn) 7, VAMP8, and Vti1-b but also with Syn13, Syn6, and the Sec1/Munc18 protein mVps45, which catalyze early endosomal fusion events. Moreover, antimVps18 antibodies inhibited early endosome fusion in vitro. Mammalian mVps18 also associated with mVam2 and …