Updated project metadata. Alpha-synuclein (αSyn) protein levels correlate with the risk and severity of Parkinson's disease and related neurodegenerative diseases. Lowering αSyn is being actively investigated as a therapeutic modality. Here we systematically map the regulatory network that controls endogenous αSyn using sequential CRISPR-knockout and -interference screens in αSyn gene(SNCA) tagged cell lines and induced pluripotent stem cell-derived neurons (iNeurons). We uncover αSyn modifiers at multiple regulatory layers, with N-terminal acetyltransferase B (NatB) enzymes being the most potent endogenous αSyn modifier in both cell lines. N-terminal acetylation protects the cytosolic αSyn from rapid degradation by the proteasome in a Ube2Wdependent manner. Moreover, we show that pharmacological inhibition of methionylaminopeptidase 2 (METAP2), a regulator of NatB complex formation, attenuates endogenous αSyn in iNeurons carrying SNCA triplication. Together, our study reveals several gene networks that control endogenous αSyn, identifies mechanisms mediating the degradation of nonacetylated αSyn and illustrates potential therapeutic pathways for decreasing αSyn levels in synucleinopathies.