A microprotein atlas of the human frontal cortex in Alzheimer's disease.

Understanding the molecular basis of neurodegeneration requires a comprehensive map of the genome's protein-coding output. Although thousands of small open reading frames (ORFs) are translated in the human brain, proteomic evidence for their encoded microproteins (MPs) (≤150 amino acids (aa)) remains limited. Here, we present a brain MP atlas that integrates transcriptomics, mass spectrometry and deep-learning-predicted spectra across more than 600 postmortem frontal cortex samples with and without Alzheimer's disease (AD). We identified 1,067 MPs absent from reviewed UniProtKB entries with high-confidence spectral support. A subset is differentially expressed in AD independently of the annotated main ORF at the same locus. A small ORF expressed by MKKS encodes a 63-amino-acid MP that is the locus's predominant translation product and is downregulated in AD; its loss impairs microglial mitochondrial respiration, implicating it in microglial bioenergetics. This atlas expands the annotated brain proteome and provides a resource for studying MPs in aging and neurodegeneration.