# Structural and functional studies of rabbit SAMD9 reveal a distinct tRNase module that underlies the antiviral activity

## TL;DR

The paper shows that rabbit SAMD9 retains antiviral and tRNA-cleaving functions despite structural differences compared to human SAMD9.

## Contribution

The study reveals that rabbit SAMD9 has an extended tRNase module and a unique residue important for antiviral activity.

## Key findings

- Rabbit SAMD9 restricts vaccinia virus replication and reduces tRNAPhe levels like human SAMD9.
- The tRNase module of rabbit SAMD9 extends beyond the conserved domain to include the SIR2 region.
- A unique basic residue in rabbit SAMD9 is important for its antiviral activity.

## Abstract

Human SAMD9 and SAMD9L (collectively SAMD9/9L) are large cytoplasmic proteins with antiviral and antiproliferative activities, recently shown to regulate protein synthesis by specifically depleting phenylalanine tRNA (tRNAPhe). The enzymatic activity of human SAMD9 (hSAMD9) resides within its N-terminal tRNase domain, which depends on three essential basic residues for tRNA binding and biological activity. While these residues are highly conserved across mammalian SAMD9/9L, lagomorph SAMD9 orthologs uniquely harbor a charge-reversal acidic residue at one of three sites, a change known to inactivate hSAMD9/9L. Here, we show that despite this variation, rabbit SAMD9 (rSAMD9) potently restricts vaccinia virus replication and specifically reduces tRNAPhe levels, mirroring hSAMD9. However, unlike hSAMD9, rSAMD9’s minimal tRNase module extends beyond the homologous tRNase domain (amino acid 158–389) to include the SIR2 region. Additional basic residues, one unique to rSAMD9, were also found to be important for its antiviral activity. The crystal structure of rSAMD9158–389 closely resembles hSAMD9156–385, though with difference in loop conformations. These findings demonstrate that lagomorph SAMD9 preserves core tRNA-targeting and antiviral functions despite a key residue variation and the need for an extended tRNase module.

Sterile Alpha Motif Domain-containing 9 (SAMD9) and its paralog SAMD9-like (SAMD9L) are antiviral factors and tumor suppressors. Gain of function (GoF) mutations of SAMD9/9L, however, cause a spectrum of human diseases with immunological or/and neurological presentations. We recently identified the effector domain of SAMD9/9L as a previously unrecognized tRNase, a function critical for both their normal physiological roles and the pathogenic effects by patient-derived mutations. We also identified three basic residues within the tRNase domain that are highly conserved across mammalian SAMD9/9L as key to its enzymatic and biological activities. However, lagomorph SAMD9 orthologs uniquely harbor a negatively charged acidic residue at one of three sites. Through extensive structural and functional analyses, here we demonstrate that rabbit SAMD9 (rSAMD9) retains both tRNA-cleaving and antiviral activities, despite notable differences in its tRNase module, including a unique basic residue specific to rSAMD9 and a larger domain architecture extending beyond the conserved tRNase core. These data highlight both the evolutionary conservation of tRNase activity in mammalian SAMD9/9L and the flexibility of the tRNase module within the protein family.