CBP-mediated SMN acetylation modulates Cajal body biogenesis and the cytoplasmic targeting of SMN

Abstract

The survival of motor neuron (SMN) protein plays an essential role in the biogenesis of spliceosomal snRNPs and the molecular assembly of Cajal bodies (CBs). Deletion of or mutations in the SMN1 gene cause spinal muscular atrophy (SMA) with degeneration and loss of motor neurons. Reduced SMN levels in SMA lead to deficient snRNP biogenesis with consequent splicing pathology. Here, we demonstrate that SMN is a novel and specific target of the acetyltransferase CBP (CREB-binding protein). Furthermore, we identify lysine (K) 119 as the main acetylation site in SMN. Importantly, SMN acetylation enhances its cytoplasmic localization, causes depletion of CBs, and reduces the accumulation of snRNPs in nuclear speckles. In contrast, the acetylation-deficient SMNK119R mutant promotes formation of CBs and a novel category of promyelocytic leukemia (PML) bodies enriched in this protein. Acetylation increases the half-life of SMN protein, reduces its cytoplasmic diffusion rate and modifies its interactome. Hence, SMN acetylation leads to its dysfunction, which explains the ineffectiveness of HDAC (histone deacetylases) inhibitors in SMA therapy despite their potential to increase SMN levels.

The role of survival motor neuron protein (SMN) in protein homeostasis

“…Post-translational modifications to the SMN protein discern its localisation and function. As well as SUMOylation via the SIM regulating Sm protein binding, and ubiquitination of SMN (discussed later), the protein may also be acetylated, which promotes a cytoplasmic localisation and increases its half-life [ 40 ], or phosphorylated on certain serine/threonine residues to promote its localisation to CBs. Mutations of three tyrosine residues in the Tudor domain greatly affect its enrichment in CBs via disrupted interaction with coilin [ 41 , 42 ].…”

Coordination of RNA Processing Regulation by Signal Transduction Pathways

“…Other evidence connects protein acetylation more directly with RNA processing regulation. For instance, acetylation of the signal transduction and activation of RNA (STAR) protein Sam68, which is involved in AS and APA [23,74,75], was shown to enhance its affinity for target RNA sequences; this PTM was increased in cancer cells [76]. More recently, it was shown that the HAT known as CREB-binding protein (CBP) is directly involved in snRNP biogenesis by promoting K119 acetylation of SMN in vivo and that the SMN acetylation status controls its subcellular localization and regulates its binding with proteins required for snRNP biogenesis [77].…”

“…On the other hand, serine/threonine phosphorylation of Sam68 by ERKs [74] or Ca2+-calmodulin kinase IV (CAMKIV) [114] promoted its splicing activity, as well as the ability of Sam68 to induce translation of target transcripts in the cytoplasm [115]. Moreover, other PTMs, such as arginine methylation and acetylation, were also shown to affect Sam68 activity by modulating its binding affinity for proteins or RNA [76,85,104,116,117].…”

“…Shuttling of SMAR1 protein from nucleus to cytoplasm disrupts the trimeric complex on the CD44 pre-mRNA and favors Sam68 acetylation [153]. Since this PTM enhances the affinity of Sam68 for RNA [74,76], RAS activation ultimately leads to increased expression of CD44 variants comprising the variable exons, which confer an invasive and metastatic phenotype to breast tumor cells (Figure 2B) [153]. Interestingly, since both SMAR1 [153] and Sam68 [148,154] interact with the snRNPs, the trimeric SMAR1-Sam68-HDAC6 complex might also act as a roadblock to sequester snRNPs and to prevent their functional interaction within the spliceosome.…”

Molecular basis and therapy of spinal muscular atrophy

“…SMN lokalizuje się w cytoplazmie i na terenie jądra komórkowego [31]. W jądrze komórkowym SMN występuje w obrębie małych struktur zwanych ciałkami Cajala [32]; ich występowanie cechuje komórki o wysokiej aktywności metabolicznej, takie jak neurony czy komórki nowotworowe [33].…”