The phospho-landscape of the survival of motoneuron protein (SMN) protein: relevance for spinal muscular atrophy (SMA)

Abstract

Spinal muscular atrophy (SMA) is caused by low levels of the survival of motoneuron (SMN) Protein leading to preferential degeneration of lower motoneurons in the ventral horn of the spinal cord and brain stem. However, the SMN protein is ubiquitously expressed and there is growing evidence of a multisystem phenotype in SMA. Since a loss of SMN function is critical, it is important to decipher the regulatory mechanisms of SMN function starting on the level of the SMN protein itself. Posttranslational modifications (PTMs) of proteins regulate multiple functions and processes, including activity, cellular trafficking, and stability. Several PTM sites have been identified within the SMN sequence. Here, we map the identified SMN PTMs highlighting phosphorylation as a key regulator affecting localization, stability and functions of SMN. Furthermore, we propose SMN phosphorylation as a crucial factor for intracellular interaction and cellular distribution of SMN. We outline the relevance of phosphorylation of the spinal muscular atrophy (SMA) gene product SMN with regard to basic housekeeping functions of SMN impaired in this neurodegenerative disease. Finally, we compare SMA patient mutations with putative and verified phosphorylation sites. Thus, we emphasize the importance of phosphorylation as a cellular modulator in a clinical perspective as a potential additional target for combinatorial SMA treatment strategies.

Keywords: Phosphorylation; Posttranslational modification (PTM); Spinal muscular atrophy (sMA); Survival of motoneuron (SMN) protein.

Fig. 1

The PTM-landscape of SMN. The SMN protein is encoded by 8 exons comprising different conserved domains. The depicted PTM sites are identified by mass spectrometry/proteomics and other methods (see also Table 1). P phosphorylation, Ac acetylation, Ub/UbUbUb mono-/polyubiquitinylation, M methylation

Fig. 2

Altered phosphorylation of the SMN protein in SMA patients. To date, there are 26 phosphorylation sites within the SMN protein sequence, which have been identified by mass spectrometry. To our knowledge, 12 point mutations of SMN found in SMA patients are putative phosphorylation sites. Only 5 of these known SMA patient mutations are also identified as phosphorylation sites. The residual 7 point mutations remain putative phosphorylation sites within the SMN protein

Fig. 3

Conservation of MS-identified phosphorylation sites of the SMN protein. The multiple alignment of the SMN protein sequences of 24 species including fungi, arthropods, nematodes, fish, birds, reptiles, amphibians and mammals was performed with the freeware CLUSTALW [79]. The conservation of the phosphorylation sites within the 8 encoding exons was further analyzed between the 24 species. The conservation of a phospho-site found in 24 of 24 species (24/24) was set as 100%. For a translational perspective the conservation of SMN phosphorylation sites between human and mouse was additionally determined