A role for polyproline motifs in the spinal muscular atrophy protein SMN. Profilins bind to and colocalize with smn in nuclear gems.
Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by the loss of alpha-motoneurons in the spinal cord followed by atrophy of skeletal muscles. SMA-determining candidate genes, SMN1 and SMN2, have been identified on human chromosome 5q. The corresponding SMN protein is expressed ubiquitously. It is coded by seven exons and contains conspicuous proline-rich motifs in its COOH-terminal third (exons 4, 5, and 6). Such motifs are known to bind to profilins (PFNs), small proteins engaged in the control of actin dynamics. We tested whether profilins interact with SMN via its polyproline stretches. Using the yeast two-hybrid system we show that profilins bind to SMN and that this binding depends on its proline-rich motifs. These results were confirmed by coimmunoprecipitation and by in vitro binding studies. Two PFN isoforms, I and II, are known, of which II is characteristic for central nervous system tissue. We show by in situ hybridization that both PFNs are highly expressed in mouse spinal cord and that PFN II is expressed predominantly in neurons. In motoneurons, the primary target of neurodegeneration in SMA, profilins are highly concentrated and colocalize with SMN in the cytoplasm of the cell body and in nuclear gems. Likewise, SMN and PFN I colocalize in gems of HeLa cells. Although SMN interacts with both profilin isoforms, binding of PFN II was stronger than of PFN I in all assays employed. Because the SMN genes are expressed ubiquitously, our findings suggest that the interaction of PFN II with SMN may be involved in neuron-specific effects of SMN mutations.
Giesemann
T
T
Department of Cell Biology, Zoological Institute, Technical University of Braunschweig, D-38092 Braunschweig, Germany.
Rathke-Hartlieb
S
S
Rothkegel
M
M
Bartsch
J W
JW
Buchmeier
S
S
Jockusch
B M
BM
Jockusch
H
H
D020816:Amino Acid Motifs; D000818:Animals; D002417:Cattle; D002467:Cell Nucleus / Q000378:metabolism*; D003285:Contractile Proteins; D017362:Cyclic AMP Response Element-Binding Protein; D006367:HeLa Cells; D006801:Humans; D007150:Immunohistochemistry; D051379:Mice; D008810:Mice, Inbred C57BL; D008840:Microfilament Proteins / Q000235:genetics / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000502:physiology*; D010455:Peptides / Q000737:chemistry*; D051304:Profilins; D011485:Protein Binding; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D016601:RNA-Binding Proteins; D055532:SMN Complex Proteins; D013116:Spinal Cord / Q000378:metabolism; D055533:Survival of Motor Neuron 1 Protein; D055540:Survival of Motor Neuron 2 Protein; D020798:Two-Hybrid System Techniques
D003285:Contractile Proteins; D017362:Cyclic AMP Response Element-Binding Protein; D008840:Microfilament Proteins; D009419:Nerve Tissue Proteins; C495738:PFN1 protein, human; D010455:Peptides; C495739:Pfn1 protein, mouse; D051304:Profilins; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D055532:SMN Complex Proteins; C526471:SMN1 protein, human; C526477:SMN2 protein, human; C526472:Smn1 protein, mouse; D055533:Survival of Motor Neuron 1 Protein; D055540:Survival of Motor Neuron 2 Protein; C011083:polyproline