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In Saccharomyces cerevisiae, the redundant YCK1 and YCK2 genes (Yeast Casein Kinase 1) are required for viability. We describe here the molecular analysis of four mutations that eliminate the requirement for Yck activity. These mutations alter proteins that resemble the four subunits of clathrin adaptors (APs), with highest sequence similarity to those of the recently identified AP-3 complex. The four yeast subunits are associated in a high-molecular-weight complex. These proteins have no essential function and are not redundant for function with other yeast AP-related proteins. Combination of suppressor mutations with a clathrin heavy chain mutation (chc1-ts) confers no synthetic growth defects. However, a yckts mutation shows a strong synthetic growth defect with chc1-ts. Moreover, endocytosis of Ste3p is dramatically decreased in yckts cells and is partially restored by the AP suppressor mutations. These results suggest that vesicle trafficking at the plasma membrane requires the activity of Yck protein kinases, and that the new AP-related complex may participate in this process. [ABSTRACT FROM AUTHOR]