1 La-related protein 1 (LARP1) binds the mRNA cap, blocking eIF4F assembly 2 on TOP mRNAs 3 Roni M. Lahr1, Bruno D. Fonseca2, Gabrielle E. Ciotti1, Hiba A. Al-Ashtal1, Jian-Jun Jia2, 4 Marius R. Niklaus2, Sarah P. Blagden3, Tommy Alain2, Andrea J. Berman1* 5 1University of Pittsburgh, Department of Biological Sciences, Pittsburgh, PA, USA. 6 2Children’s Hospital of Eastern Ontario Research Institute, Ottawa, ON, Canada. 7 3University of Oxford, Department of Oncology, Oxford UK. 8 *Correspondence to:
[email protected]. 9 10 Abstract: 11 The 5’terminal oligopyrimidine (5’TOP) motif is a cis-regulatory RNA element located 12 immediately downstream of the 7-methyl-guanosine [m7G] cap of TOP mRNAs, which encode 13 ribosomal proteins and translation factors. In eukaryotes, this motif coordinates the synchronous 14 and stoichiometric expression of the protein components of the translation machinery. La-related 15 protein 1 (LARP1) binds TOP mRNAs, regulating their stability and translation. We present 16 crystal structures of the human LARP1 DM15 region in complex with a 5’TOP motif, a cap 17 analog (m7GTP), and a capped cytosine (m7GpppC) resolved to 2.6, 1.8 and 1.7 Å, respectively. 18 Our binding, competition, and immunoprecipitation data corroborate and elaborate on the 19 mechanism of 5’TOP motif binding by LARP1. We show that LARP1 directly binds the cap and 20 adjacent 5’TOP motif of TOP mRNAs, effectively impeding access of eIF4E to the cap and 21 preventing eIF4F assembly. Thus, LARP1 is a specialized TOP mRNA cap-binding protein that 22 controls ribosome biogenesis.