Proteogenomic Profiling Reveals Small ORFs and Functional Microproteins in Activated T Cells

Noncanonical micropeptides or called novel microproteins, i.e., polypeptides mostly under 10 kDa, are encoded by genomic sequences that have been previously annotated as noncoding but now known as small open reading frames (sORFs). The recent identification of microproteins encoded by sORFs has provided evidence that many sORFs encode functional microproteins that play crucial roles in various biological processes. T cell activation is a critical biological process for adaptive immune response. Understanding key players in this process will allow us to decipher the complex mechanisms as well as develop immunotherapy for treating a wide range of diseases. Although there have been extensive studies on canonical proteins in T cell activation, the novel microproteins in T cells and their roles have been uncharted water to date. Nascent proteins are defined as newly synthesized polypeptides emerged during the translation of mRNA. In this study, we combined nascent proteomics and quantitative proteomics to identify 411 novel microproteins in primary human T cells, including 83 nascent microproteins. We activated the T cell function with either PMA/Ionomycin (distal activation) or CD3/CD28 activating Antibodies (proximal activation), and obtained a comprehensive canonical protein and microprotein profiles to pinpoint common and distinct differentially expressed proteins under these two activation conditions. After experimental testing, three microproteins numbered T1, T2 and T3 were found to be functional in regulating T cell activation. Bioinformatic and proteomic analyses suggested that T1 was functional related to immune as negative feedback to T cell activation. Our study not only established an integrated approach to uncover and elucidate novel microproteins but also highlight the significant role of microproteins in regulating T cell activation.