Genetic efficiency in higher organisms depends on mechanisms to create multiple functions from single genes. To investigate this question for an enzyme family, we chose aminoacyl tRNA synthetases (AARSs). They are exceptional in their progressive and accretive proliferation of noncatalytic domains as the Tree of Life is ascended. Here we report discovery of a large number of natural catalytic nulls (CNs) for each human AARS. Splicing events retain noncatalytic domains while ablating the catalytic domain (CD) to create CNs with diverse functions. Each synthetase is converted into several new signaling proteins with biological activities ‘orthogonal’ to that of the catalytic parent. We suggest that splice variants with non-enzymatic functions may be more general, as evidenced by recent findings of other catalytically inactive splice-variant enzymes.

The tRNA synthetases catalyze the first step of protein synthesis and have increasingly been studied for their nuclear and extra-cellular ex-translational activities. Human genetic conditions such as Charcot-Marie-Tooth have been attributed to dominant gain-of-function mutations in some tRNA synthetases. Unlike dominantly inherited gain-of-function mutations, recessive loss-of-function mutations can potentially elucidate ex-translational activities. We present here five individuals from four families with a multi-system disease associated with bi-allelic mutations in FARSB that encodes the beta chain of the alpha2beta2 phenylalanine-tRNA synthetase (FARS). Collectively, the mutant alleles encompass a 5′-splice junction non-coding variant (SJV) and six missense variants, one of which is shared by unrelated individuals. The clinical condition is characterized by interstitial lung disease, cerebral aneurysms and brain calcifications, and cirrhosis. For the SJV, we confirmed exon skipping leading to a frameshift associated with noncatalytic activity. While the bi-allelic combination of the SJV with a p.Arg305Gln missense mutation in two individuals led to severe disease, cells from neither the asymptomatic heterozygous carriers nor the compound heterozygous affected individual had any defect in protein synthesis. These results support a disease mechanism independent of tRNA synthetase activities in protein translation and suggest that this FARS activity is essential for normal function in multiple organs.

Background: Autoantibodies (anti-Jo-1) to cytoplasmic histidyl-tRNA synthetase (HisRS) are associated with inflammatory myositis. Results: HisRS and two splice variants (SVs) cross-react with anti-Jo-1 antibodies and are secreted; at least one SV transcript is up-regulated in dermatomyositis. Conclusion: Secreted HisRS SVs contain major epitopes of anti-Jo-1 autoantibodies. Significance: Secreted HisRS and its SVs share epitopes for potential extracellular anti-Jo-1 antibody binding.