Effects of the Bowen-Conradi syndrome mutation in EMG1 on its nuclear import, stability and nucleolar recruitment

Bowen-Conradi syndrome (BCS) is a severe genetic disorder that is characterised by various developmental abnormalities, bone marrow failure and early infant death. This disease is caused by a single mutation leading to the aspartate 86 to glycine (D86G) exchange in the essential nucleolar RNA methyltransferase EMG1. EMG1 is required for the synthesis of the small ribosomal subunit and is involved in modification of the 18S ribosomal RNA. Here, we identify the pre-ribosomal factors NOP14, NOC4L and UTP14A as members of a nucleolar subcomplex that contains EMG1 and is required for its recruitment to nucleoli. The BCS mutation in EMG1 leads to reduced nucleolar localisation, accumulation of EMG1(D86G) in nuclear foci and its proteasome-dependent degradation. We further show that EMG1 can be imported into the nucleus by the importins (Imp) Impα/β or Impβ/7. Interestingly, in addition to its role in nuclear import, binding of the Impβ/7 heterodimer can prevent unspecific aggregation of both EMG1 and EMG1(D86G) on RNAs in vitro, indicating that the importins act as chaperones by binding to basic regions of the RNA methyltransferase. Our findings further indicate that in BCS, nuclear disassembly of the import complex and release of EMG1(D86G) lead to its nuclear aggregation and degradation, resulting in the reduced nucleolar recruitment of the RNA methyltransferase and defects in the biogenesis of the small ribosomal subunit.