THE YEAST ACTIN-RELATED PROTEIN ARP2P IS REQUIRED FOR THE INTERNALIZATION STEP OF ENDOCYTOSIS

The Saccharomyces cerevisiae actin-related protein Arp2p is an essenti al component of the actin cytoskeleton. We have tested its potential r ole in the endocytic and exocytic pathways by using a temperature-sens itive allele, arp2-1. The fate of the plasma membrane transporter urac il permease was followed to determine whether Arp2p plays a role in th e endocytic pathway Inhibition of normal endocytosis as revealed by ma intenance of active uracil permease at the plasma membrane and strong protection against subsequent vacuolar degradation of the protein were observed in the mutant at the restrictive temperature. Furthermore, a rp2-1 cells accumulated ubiquitin-permease conjugates, formed prior to internalization. These effects were also visible at permissive temper ature, whereas the actin cytoskeleton appeared to be normally polarize d. The soluble hydrolase carboxypeptidase Y and the lipophilic dye FM 4-64 were targeted normally to the vacuole in arp2-1 cells. Thus, Arp2 p is required for internalization but does not play a major role in la ter steps of endocytosis. Synthetic lethality was demonstrated between arp2-1 and the endocytic mutant end3-1, suggesting participation of A rp2p and End3p in the same process. Finally, no evidence for a major d efect in secretion was apparent; invertase secretion and delivery of u racil permease to the plasma membrane were unaffected in arp2-1 cells.