The SMN Interactome Includes Myb-Binding Protein 1a

Understanding networks of interacting proteins is a major goal in cell biology. The survival of motor neurons protein (SMN) interacts, directly or indirectly, with a large number of other proteins and reduced levels of SMN cause the inherited disorder spinal muscular atrophy (SMA). Some SMN interactions are stable and stoichiometric, such as those with gemins, while others are expected to be transient and substoichiometric, such as the functional interaction of SMN with coilin in Cajal bodies. This study set out to determine whether novel components of the extensive SMN interactome can be identified by a proteomic approach. SMN complexes were immuno-precipitated from HeLa nuclear extracts, using anti-SMN monoclonal antibody attached to magnetic beads, digested with trypsin, separated by capillary-liquid chromatography and analyzed by MALDI TOF/TOF mass spectrometry. One-hundred and one proteins were detected with a p value of <0.05, SMN, gemins and U snRNPs being the dominant "hits". Sixty-nine of these were rejected after MALDI analysis of two control pull-downs using antibodies against unrelated nuclear proteins. The proteins found only in anti-SMN pulldowns were either known SMN partners, and/or contained dimethylated RG domains involved in direct interaction with the SMN tudor domain, or they were known binding partners of such direct SMN interactors. Myb-binding protein 1 a, identified as a novel candidate, is a mainly nucleolar protein of unknown function but it partially colocalized with SMN in Cajal bodies in HeLa cell nucleoplasm and, like SMN, was reduced in cells from an SMA patient.