Protein kinase C (PKC) isozymes function as tumor suppressors in increasing contexts. In contrast to oncogenic kinases, whose function is acutely regulated by transient phosphorylation, PKC is constitutively phosphorylated following biosynthesis to yield a stable, autoinhibited enzyme that is reversibly activated by second messengers. Here, we report that the phosphatase PHLPP1 opposes PKC phosphorylation during maturation, leading to the degradation of aberrantly active species that do not become autoinhibited. Cancer-associated hotspot mutations in the pseudosubstrate of PKCβ that impair autoinhibition result in dephosphorylated and unstable enzymes. Protein-level analysis reveals that PKCα is fully phosphorylated at the PHLPP site in over 5,000 patient tumors, with higher PKC levels correlating (1) inversely with PHLPP1 levels and (2) positively with improved survival in pancreatic adenocarcinoma. Thus, PHLPP1 provides a proofreading step that maintains the fidelity of PKC autoinhibition and reveals a prominent loss-of-function mechanism in cancer by suppressing the steady-state levels of PKC. • Phosphorylation of newly synthesized PKC is necessary for stabilizing autoinhibition • PHLPP1 dephosphorylates newly synthesized PKC to provide quality control • PKC quality control is conserved in >5,000 patient samples from 19 cancers • Pancreatic cancer patients with high PKC and low PHLPP1 have improved survival PKC generally functions as a tumor suppressor. Baffi et al. uncover a quality control mechanism in which PHLPP1 opposes priming phosphorylation of newly synthesized PKC to suppress its steady-state levels. This quality control dominates in pancreatic cancer: patients with high levels of PHLPP1 and low levels of PKC have worsened survival. [ABSTRACT FROM AUTHOR]