The integrin-adhesome is required to maintain muscle structure, mitochondrial ATP production, and movement forces in Caenorhabditis elegans.

The integrin-adhesome network, which contains >150 proteins, is mechano-transducing and located at discreet positions along the cell-cell and cell-extracellular matrix interface. A small subset of the integrin-adhesome is known to maintain normal muscle morphology. However, the importance of the entire adhesome for muscle structure and function is unknown. We used RNA interference to knock down 113 putative Caenorhabditis elegans homologs constituting most of the mammalian adhesome and 48 proteins known to localize to attachment sites in C. elegans muscle. In both cases, we found >90% of components were required for normal muscle mitochondrial structure and/or proteostasis vs. empty vector controls. Approximately half of these, mainly proteins that physically interact with each other, were also required for normal sarcomere and/or adhesome structure. Next we confirmed that the dystrophy observed in adhesome mutants associates with impaired maximal mitochondrial ATP production (P < 0.01), as well as reduced probability distribution of muscle movement forces compared with wild-type animals. Our results show that the integrin-adhesome network as a whole is required for maintaining both muscle structure and function and extend the current understanding of the full complexities of the functional adhesome in vivo.

Mutations in the survival motor neuron (SMN) protein alter the dynamic nature of nuclear bodies.

The childhood disorder spinal muscular atrophy (SMA) is caused by reduced expression of the survival motor neuron (SMN) protein. SMN is a multifunctional protein that has been implicated in the production, processing and transport of RNA and ribonucleoproteins (RNPs). Within the nucleus, SMN is predominantly targeted to Cajal bodies (CB), which are involved in the maturation and processing of several subclasses of RNPs. Here, we show that the SMN exon 2b-encoded domain (SMN2b) is independently sufficient to mediate CB targeting, but that the resulting bodies are less dynamic than those containing full-length SMN protein. We also show that while two SMN proteins harbouring SMA-causing point mutations (A2G and S262I) are efficiently targeted to CBs, they also display reduced nuclear movement.

A mechanism of release of calreticulin from cells during apoptosis.

Calreticulin (CRT) is an endoplasmic reticulum (ER) chaperone responsible for glycoprotein folding and Ca(2+) homeostasis. CRT also has extracellular functions, e.g. tumor and apoptotic cell recognition and wound healing, but the mechanism of CRT extracellular release is unknown. Cytosolic localization of CRT is determined by signal peptide and subsequent retrotranslocation of CRT into the cytoplasm. Here, we show that under apoptotic stress conditions, the cytosolic concentration of CRT increases and associates with phosphatidylserine (PS) in a Ca(2)(+)-dependent manner. PS distribution is regulated by aminophospholipid translocase (APLT), which maintains PS on the cytosolic side of the cell membrane. APLT is sensitive to redox modifications of its SH groups by reactive nitrogen species. During apoptosis, both CRT expression and the concentration of nitric oxide (NO) increase. By using S-nitroso-l-cysteine-ethyl-ester, an intracellular NO donor and inhibitor of APLT, we showed that PS and CRT externalization occurred together in an S-nitrosothiol-dependent and caspase-independent manner. Furthermore, the CRT and PS are relocated as punctate clusters on the cell surface. Thus, CRT induced nitrosylation and its externalization with PS could explain how CRT acts as a bridging molecule during apoptotic cell clearance.

SMN, Gemin2 and Gemin3 associate with beta-actin mRNA in the cytoplasm of neuronal cells in vitro.

Childhood spinal muscular atrophy is caused by a reduced expression of the survival motor neuron (SMN) protein. SMN has been implicated in the axonal transport of beta-actin mRNA in both primary and transformed neuronal cell lines, and loss of this function could account, at least in part, for spinal muscular atrophy onset and pathological specificity. Here we have utilised a targeted screen to identify mRNA associated with SMN, Gemin2 and Gemin3 in the cytoplasm of a human neuroblastoma cell line, SHSY5Y. Importantly, we have provided the first direct evidence that beta-actin mRNA is present in SMN cytoplasmic complexes in SHSY5Y cells.

SMN and the Gemin proteins form sub-complexes that localise to both stationary and dynamic neurite granules.

Childhood spinal muscular atrophy (SMA) is caused by a reduction in survival motor neuron (SMN) protein. SMN is expressed in every cell type, but it is predominantly the lower motor neurones of the spinal cord that degenerate in SMA. SMN has been linked to the axonal transport of beta-actin mRNA, a breakdown in which could trigger disease onset. It is known that SMN is present in transport ribonucleoproteins (RNPs) granules that also contain Gemin2 and Gemin3. To further characterise these granules we have performed live cell imaging of GFP-tagged SMN, GFP-Gemin2, GFP-Gemin3, GFP-Gemin6 and GFP-Gemin7. In all, we have made two important observations: (1) SMN granules appear metamorphic; and (2) the SMN-Gemin complex(es) appears to localise to two distinct subsets of bodies in neurites; stationary bodies and smaller dynamic bodies. This study provides an insight into the neuronal function of the SMN complex.

Identification of a self-association domain in the Ewing's sarcoma protein: a novel function for arginine-glycine-glycine rich motifs?

The Ewing's sarcoma (EWS) protein is a ubiquitously expressed RNA chaperone. The EWS protein localizes predominantly to the nucleus. Previous reports have suggested that the EWS protein is capable of dimerizing. However, to date this has not been confirmed. Here, using a novel panel of recombinant proteins, we have performed an in vitro biomolecular interaction analysis of the EWS protein. We have demonstrated that all three arginine-glycine-glycine (RGG) motifs are capable of binding directly to the survival motor neuron protein, a Tudor domain containing EWS binding partner. We have also confirmed EWS is capable of self-associating, and we have mapped this binding domain to the RGG motifs. We have also found that self-association may be required for EWS nuclear import. This is the first direct evidence of RGG domains being involved in self-association and has implications on all RGG-containing proteins.

Identification of a tripartite import signal in the Ewing Sarcoma protein (EWS).

The Ewing Sarcoma (EWS) protein is a ubiquitously expressed RNA processing factor that localises predominantly to the nucleus. However, the mechanism through which EWS enters the nucleus remains unclear, with differing reports identifying three separate import signals within the EWS protein. Here we have utilized a panel of truncated EWS proteins to clarify the reported nuclear localisation signals. We describe three C-terminal domains that are important for efficient EWS nuclear localization: (1) the third RGG-motif; (2) the last 10 amino acids (known as the PY-import motif); and (3) the zinc-finger motif. Although these three domains are involved in nuclear import, they are not independently capable of driving the efficient import of a GFP-moiety. However, collectively they form a complex tripartite signal that efficiently drives GFP-import into the nucleus. This study helps clarify the EWS import signal, and the identification of the involvement of both the RGG- and zinc-finger motifs has wide reaching implications.

Targeting of SMN to Cajal bodies is mediated by self-association.

The childhood autosomal recessive disorder spinal muscular atrophy (SMA) is caused by mutations in the survival motor neuron (SMN) gene. SMN localizes diffusely in the cytoplasm and in distinct nuclear structures called Cajal bodies. Cajal bodies are believed to be the storage and processing sites of several ribonucleoproteins. Here, using a novel panel of SMN exon deletion constructs, we report a systematic analysis of internal targeting domains in the SMN protein. We demonstrate that the peptides encoded by exons 2b, 3 and 6 perform an integral role in the cellular targeting of SMN. In addition, we identify a nine amino acid motif within the highly conserved sequences of the exon 2b encoded domain that mediates Cajal body targeting and self-association. Deletion of this domain dramatically affects SMN activity and results in a dominant-negative clone. These results identify critical domains within the SMN protein and have an impact on our understanding of the SMN protein with regards to SMA as well as cellular biology.