Structural and Functional Impact of SRP54 Mutations Causing Severe Congenital Neutropenia

The SRP54 GTPase is a key component of co-translational protein targeting by the signal recognition particle (SRP). Point mutations in SRP54 have been recently shown to lead to a form of severe congenital neutropenia displaying symptoms overlapping with those of Shwachman-Diamond syndrome. The phenotype includes severe neutropenia, exocrine pancreatic deficiency, and neurodevelopmental as well as skeletal disorders. Using a combination of X-ray crystallography, hydrogen-deuterium exchange coupled to mass spectrometry and complementary biochemical and biophysical methods, we reveal extensive structural defects in three disease-causing SRP54 variants resulting in critical protein destabilization. GTP binding is mostly abolished as a consequence of an altered GTPase core. The mutations located in conserved sequence fingerprints of SRP54 eliminate targeting complex formation with the SRP receptor as demonstrated in yeast and human cells. These specific defects critically influence the entire SRP pathway, thereby causing this life-threatening disease. [Display omitted] •The pathogenic SRP54 mutations studied destabilize the SRP54 G domain•SRP54 complex formation with the SRP receptor is abolished in mutant variants•GTP binding by SRP54 mutant variants leads to differential effects•SRP54 mutations negatively impact SRP-mediated protein secretion Juaire et al. demonstrate that mutations in SRP54, which are linked to severe congenital neutropenia, lead to critically destabilized regions in SRP54 and increase overall protein flexibility. Furthermore, the mutations negatively impact GTP binding and complex formation with the SRP receptor, thereby severely affecting protein secretion by the SRP pathway.