Protein S-palmitoylation is a posttranslational modification in which a palmitoyl group is added to a protein via a thioester linkage on cysteine. Palmitoylation is a reversible modification involved in protein membrane targeting, receptor trafficking and signaling, vesicular biogenesis and trafficking, protein aggregation, and protein degradation. An example of the dynamic nature of this modification is the palmitoylation-depalmitoylation cycle that regulates the subcellular trafficking of Ras family GTPases. The Ras protein acyltransferase (PAT) consists of a complex of Erf2-Erf4 and DHHC9-GCP16 in yeast and mammalian cells, respectively. Both subunits are required for PAT activity, but the function of the Erf4 and Gcp16 subunits has not been established. This study elucidates the function of Erf4 and shows that one role of Erf4 is to regulate Erf2 stability through an ubiquitin-mediated pathway. In addition, Erf4 is required for the stable formation of the palmitoyl-Erf2 intermediate, the first step of palmitoyl transfer to protein substrates. In the absence of Erf4, the rate of hydrolysis of the active site palmitoyl thioester intermediate is increased, resulting in reduced palmitoyl transfer to a Ras2 substrate. This is the first demonstration of regulation of a DHHC PAT enzyme by an associated protein.