The ribosomal protein eL21 interacts with the protein lysine methyltransferase SMYD2 and regulates its steady state levels.

The protein lysine methyltransferase, SMYD2 is involved in diverse cellular events by regulating protein functions through lysine methylation. Though several substrate proteins of SMYD2 are well-studied, only a limited number of its interaction partners have been identified and characterized. Here, we performed a yeast two-hybrid screening of SMYD2 and found that the ribosomal protein, eL21 could interact with SMYD2. SMYD2-eL21 interaction in the human cells was confirmed by immunoprecipitation methods. In vitro pull-down assays revealed that SMYD2 interacts with eL21 directly through its SET and MYND domain. Computational mapping, followed by experimental studies identified that Lys81 and Lys83 residues of eL21 are important for the SMYD2-eL21 interaction. Evolutionary analysis showed that these residues might have co-evolved with the emergence of SMYD2. We found that eL21 regulates the steady state levels of SMYD2 by promoting its transcription and inhibiting its proteasomal degradation. Importantly, SMYD2-eL21 interaction plays an important role in regulating cell proliferation and its dysregulation might lead to tumorigenesis. Our findings highlight a novel extra-ribosomal function of eL21 on regulating SMYD2 levels and imply that ribosomal proteins might regulate wide range of cellular functions through protein-protein interactions in addition to their core function in translation.

PRMT7 Interacts with ASS1 and Citrullinemia Mutations Disrupt the Interaction.

Protein arginine methyltransferase 7 (PRMT7) catalyzes the introduction of monomethylation marks at the arginine residues of substrate proteins. PRMT7 plays important roles in the regulation of gene expression, splicing, DNA damage, paternal imprinting, cancer and metastasis. However, little is known about the interaction partners of PRMT7. To address this, we performed yeast two-hybrid screening of PRMT7 and identified argininosuccinate synthetase (ASS1) as a potential interaction partner of PRMT7. We confirmed that PRMT7 directly interacts with ASS1 using pull-down studies. ASS1 catalyzes the rate-limiting step of arginine synthesis in urea cycle and citrulline-nitric oxide cycle. We mapped the interface of PRMT7-ASS1 complex through computational approaches and validated the predicted interface in vivo by site-directed mutagenesis. Evolutionary analysis revealed that the ASS1 residues important for PRMT7-ASS1 interaction have co-evolved with PRMT7. We showed that ASS1 mutations linked to type I citrullinemia disrupt the ASS1-PRMT7 interaction, which might explain the molecular pathogenesis of the disease.