PROJECT SUMMARY/ABSTRACT Secreted and membrane proteins are important for cell survival and crosstalk. The rough endoplasmic reticulum (ER) is a dynamic organelle that adjusts to changes in protein folding demand by regulating secretory protein throughput. While recent studies examined transcriptional regulation of genes encoding secretory proteins in the heart, post-transcriptional regulation of protein expression remains largely unstudied. Here, post-transcriptional regulation genes encoding secretory proteins will be studied, which we found to be an important layer of gene expression control. ER-associated RNA-binding proteins that bind targeting elements in transcripts may orchestrate post-transcriptional regulation of gene expression at the ER. Our pilot data identified ribosome-binding protein 1 (RRBP1) as a novel protein that binds RNA at the ER of cardiac myocytes, which we found to be necessary for secretory protein expression. We demonstrated that RRBP1 increased in cardiac disease settings that affect the ER secretory capacity. We also revealed that RRBP1 expression decreased during postnatal development, coordinate with decline in ER secretory capacity. When RRBP1 expression was increased in cardiac myocytes, transcripts encoding ER-targeted proteins increased and cardiac structure and function after myocardial infarction improved. Our concept is that RRBP1 localizes specific mRNAs that encode secretory proteins to the ER, and that this localization is important for the synthesis of proteins that improve ER secretory capacity, as well as secreted proteins. This has led to the hypothesis that RRBP1 mechanistically links post-transcriptional control of secretory protein expression and ER secretory capacity, which we posit to be adaptive in the heart. This hypothesis will be addressed in three specific aims: 1) Assess the effects of RRBP1 on mRNA metabolism, 2) Examine the effects of RRBP1 on secretory protein synthesis and cellular crosstalk, and 3) Determine the protective potential of increasing RRBP1. These studies will be performed using an innovative molecular strategy for endogenous re-expression of a gene that is downregulated specifically in cardiac myocytes to mechanistically dissect roles for RRBP1 as a regulator of gene expression and response to injury. Impact: Coupling this strategy with cardiac myocyte- specific RNA profiling and ribosome profiling will enable us to determine the effects of RRBP1 on all aspects of RNA metabolism dynamics in cardiac myocytes, in vivo. These studies will reveal previously unappreciated roles for RRBP-1 mediated post-transcriptional regulation of ER-localized mRNAs in cardiac myocyte viability and cardioprotection.