Mechanisms and functions of RPL3L ribosomes

RPL3L, or ribosomal protein L3-like, is a highly conserved, vertebrate-specific duplicated paralog of the ubiquitous ribosomal protein L3 (RPL3). While most ribosomal protein gene duplications are silent pseudogenes, RPL3L is highly and exclusively expressed in adult striated muscle and is the predominant form in cardiac (~95%) and skeletal (~60%) muscle. Such a tissue-specific paralog switching is intriguing especially given that striated muscle comprise ~50% of body weight and 50-75% of all body protein, and that RPL3 is one of the most important ribosomal proteins: it is the closest protein to ribosome catalytic center, functions as a gatekeeper to the decoding center, and is required for the first step of 60S subunit assembly. Furthermore, the development- and tissue-specific RPL3-to-RPL3L switch is reversed during muscle growth when the demand for protein synthesis is high. We and others recently reported bi-allelic heterozygous missense mutations in RPL3L co- segregating with severe neonatal dilated cardiomyopathy and fatal heart failure in five unrelated families. Despite the evolutionary, physiological and pathological significance, it remains unclear why the genetic code in half of human body mass is interpreted by ribosomes differ by a key component, i.e., RPL3 vs. RPL3L, in a highly regulated manner. The mechanistic details of whether and how RPL3L ribosomes function differently from ubiquitous RPL3 ribosomes remains poorly understood, due in part to the lack of in vitro and in vivo systems where the relative expression of RPL3 and RPL3L are perturbed. To address this barrier, we have generated human cell lines and mouse models predominantly expressing either RPL3 or RPL3L in cardiomyocytes. Our preliminary results support a model that RPL3L ribosomes promote protein homeostasis in striated muscle cells. We will use a combination of biochemical, imaging, genomics, and computational approaches to test our hypothesis. By answering the long-standing question of why the heart needs a special ribosome, our study will reveal how heart functions at the molecular and cellular level and establish a new paradigm for how ribosomes specialize in tissues. From the disease perspective, our study may shed lights on the pathogenesis of dilated cardiomyopathy and lay the foundation for the development of new therapies.