Regulation of mRNA translation elongation impacts nascent protein synthesis and integrity and plays a critical role in disease establishment. Here, we investigate features linking regulation of codon-dependent translation elongation to protein expression and homeostasis. Using knockdown models of enzymes that catalyze the mcm⁵s² wobble uridine tRNA modification (U₃₄-enzymes), we show that gene codon content is necessary but not sufficient to predict protein fate. While translation defects upon perturbation of U₃₄-enzymes are strictly dependent on codon content, the consequences on protein output are determined by other features. Specific hydrophilic motifs cause protein aggregation and degradation upon codon-dependent translation elongation defects. Accordingly, the combination of codon content and the presence of hydrophilic motifs define the proteome whose maintenance relies on U₃₄-tRNA modification. Together, these results uncover the mechanism linking wobble tRNA modification to mRNA translation and aggregation to maintain proteome homeostasis.

While melanoma cells often express a high burden of mutated proteins, the infiltration of reactive T cells rarely results in tumor-eradicating immunity. We discovered that large extracellular vesicles, known as melanosomes, secreted by melanoma cells are decorated with major histocompatibility complex (MHC) molecules that stimulate CD8⁺ T cells through their T cell receptor (TCR), causing T cell dysfunction and apoptosis. Immunopeptidomic and T cell receptor sequencing (TCR-seq) analyses revealed that these melanosomes carry MHC-bound tumor-associated antigens with higher affinity and immunogenicity, which compete with their tumor cell of origin for direct TCR-MHC interactions. Analysis of biopsies from melanoma patients confirmed that melanosomes trap infiltrating lymphocytes, induce partial activation, and decrease CD8⁺ T cell cytotoxicity. Inhibition of melanosome secretion in vivo significantly reduced tumor immune evasion. These findings suggest that MHC export protects melanoma from the cytotoxic effects of T cells. Our study highlights a novel immune evasion mechanism and proposes a therapeutic avenue to enhance tumor immunity.