Understanding the relationship between the 32-item motor function measure and daily activities from an individual with spinal muscular atrophy and their caregivers' perspective: a two-part study.

BACKGROUND: The 32-item Motor Function Measure (MFM32) is a clinician-reported outcome measure used to assess the functional abilities of individuals with neuromuscular diseases, including those with spinal muscular atrophy (SMA). This two-part study explored the relationship between the functional abilities assessed in the MFM32 and activities of daily living (ADLs) from the perspective of individuals with Type 2 and Type 3 (non-ambulant and ambulant) SMA and their caregivers through qualitative interviews and a quantitative online survey. METHODS: In-depth, semi-structured, qualitative interviews were conducted with individuals with SMA and caregivers from the US. Subsequently, a quantitative online survey was completed by individuals with SMA or their caregivers from France, Germany, Italy, Poland, Spain, Canada, the United States (US) and the UK. In both parts of the study, participants were asked to describe the ADLs considered to be related to the functional abilities assessed in the MFM32. Results from the qualitative interviews informed the content of the quantitative online survey. RESULTS: Qualitative interviews were conducted with 15 adult participants, and 217 participants completed the quantitative online survey. From the qualitative interviews, all of the functional abilities assessed in the patient-friendly MFM32 were deemed as related to one or more ADL. The specific ADLs that participants considered related to the patient-friendly MFM32 items could be grouped into 10 key ADL domains: dressing, mobility/transferring, self-care, self-feeding, reaching, picking up and holding objects, physical activity, writing and technology use, social contact/engagement, toileting and performing work/school activities. These results were confirmed by the quantitative online survey whereby the ADLs reported to be related to each patient-friendly MFM32 item were consistent and could be grouped into the same 10 ADL domains. CONCLUSION: This study provides in-depth evidence from the patient/caregiver perspective supporting the relevance of the patient-friendly MFM32 items to the ADLs of individuals with Type 2 and Type 3 SMA.

Refined topology model of the STT3/Stt3 protein subunit of the oligosaccharyltransferase complex.

The oligosaccharyltransferase complex, localized in the endoplasmic reticulum (ER) of eukaryotic cells, is responsible for the N-linked glycosylation of numerous protein substrates. The membrane protein STT3 is a highly conserved part of the oligosaccharyltransferase and likely contains the active site of the complex. However, understanding the catalytic determinants of this system has been challenging, in part because of a discrepancy in the structural topology of the bacterial versus eukaryotic proteins and incomplete information about the mechanism of membrane integration. Here, we use a glycosylation mapping approach to investigate these questions. We measured the membrane integration efficiency of the mouse STT3-A and yeast Stt3p transmembrane domains (TMDs) and report a refined topology of the N-terminal half of the mouse STT3-A. Our results show that most of the STT3 TMDs are well inserted into the ER membrane on their own or in the presence of the natural flanking residues. However, for the mouse STT3-A hydrophobic domains 4 and 6 and yeast Stt3p domains 2, 3a, 3c, and 6 we measured reduced insertion efficiency into the ER membrane. Furthermore, we mapped the first half of the STT3-A protein, finding two extra hydrophobic domains between the third and the fourth TMD. This result indicates that the eukaryotic STT3 has 13 transmembrane domains, consistent with the structure of the bacterial homolog of STT3 and setting the stage for future combined efforts to interrogate this fascinating system.

The Sec62-Sec63 translocon facilitates translocation of the C-terminus of membrane proteins.

The Sec62-Sec63 complex mediates post-translational translocation of a subset of primarily secretory proteins into the endoplasmic reticulum (ER) in yeast. Therefore, it has been thought that membrane proteins, which are mainly co-translationally targeted into the ER, are not handled by the Sec62-Sec63 translocon. By systematic analysis of single and multi-spanning membrane proteins with broad sequence context [with differing hydrophobicity, flanking charged residues and orientation of transmembrane (TM) segments], we show that mutations in the N-terminal cytosolic domain of yeast Sec62 impair its interaction with Sec63 and lead to defects in membrane insertion and translocation of the C-terminus of membrane proteins. These results suggest that there is an unappreciated function of the Sec62-Sec63 translocon in regulating topogenesis of membrane proteins in the eukaryotic cell.

Structural and functional profiling of the lateral gate of the Sec61 translocon.

The evolutionarily conserved Sec61 translocon mediates the translocation and membrane insertion of proteins. For the integration of proteins into the membrane, the Sec61 translocon opens laterally to the lipid bilayer. Previous studies suggest that the lateral opening of the channel is mediated by the helices TM2b and TM7 of a pore-forming subunit of the Sec61 translocon. To map key residues in TM2b and TM7 in yeast Sec61 that modulate lateral gating activity, we performed alanine scanning and in vivo site-directed photocross-linking experiments. Alanine scanning identified two groups of critical residues in the lateral gate, one group that leads to defects in the translocation and membrane insertion of proteins and the other group that causes faster translocation and facilitates membrane insertion. Photocross-linking data show that the former group of residues is located at the interface of the lateral gate. Furthermore, different degrees of defects for the membrane insertion of single- and double-spanning membrane proteins were observed depending on whether the mutations were located in TM2b or TM7. These results demonstrate subtle differences in the molecular mechanism of the signal sequence binding/opening of the lateral gate and membrane insertion of a succeeding transmembrane segment in a polytopic membrane protein.

Sec62 protein mediates membrane insertion and orientation of moderately hydrophobic signal anchor proteins in the endoplasmic reticulum (ER).

Nascent chains are known to be targeted to the endoplasmic reticulum membrane either by a signal recognition particle (SRP)-dependent co-translational or by an SRP-independent post-translational translocation route depending on signal sequences. Using a set of model and cellular proteins carrying an N-terminal signal anchor sequence of controlled hydrophobicity and yeast mutant strains defective in SRP or Sec62 function, the hydrophobicity-dependent targeting efficiency and targeting pathway preference were systematically evaluated. Our results suggest that an SRP-dependent co-translational and an SRP-independent post-translational translocation are not mutually exclusive for signal anchor proteins and that moderately hydrophobic ones require both SRP and Sec62 for proper targeting and translocation to the endoplasmic reticulum. Further, defect in Sec62 selectively reduced signal sequences inserted in an N(in)-C(out) (type II) membrane topology, implying an undiscovered role of Sec62 in regulating the orientation of the signal sequence in an early stage of translocation.

A short C-terminal tail prevents mis-targeting of hydrophobic mitochondrial membrane proteins to the ER.

Sdh3/Shh3, a subunit of mitochondrial succinate dehydrogenase, contains transmembrane domains with a hydrophobicity comparable to that of endoplasmic reticulum (ER) proteins. Here, we show that a C-terminal reporter fusion to Sdh3/Shh3 results in partial mis-targeting of the protein to the ER. This mis-targeting is mediated by the signal recognition particle (SRP) and depends on the length of the C-terminal tail. These results imply that if nuclear-encoded mitochondrial proteins contain strongly hydrophobic transmembrane domains and a long C-terminal tail, they have the potential to be recognized by SRP and mis-targeted to the ER.

Analysis of transmembrane helix integration in the endoplasmic reticulum in S. cerevisiae.

What sequence features in integral membrane proteins determine which parts of the polypeptide chain will form transmembrane alpha-helices and which parts will be located outside the lipid bilayer? Previous studies on the integration of model transmembrane segments into the mammalian endoplasmic reticulum (ER) have provided a rather detailed quantitative picture of the relation between amino acid sequence and membrane-integration propensity for proteins targeted to the Sec61 translocon. We have now carried out a comparative study of the integration of N out-C in-orientated 19-residue-long polypeptide segments into the ER of the yeast Saccharomyces cerevisiae. We find that the 'threshold hydrophobicity' required for insertion into the ER membrane is very similar in S. cerevisiae and in mammalian cells. Further, when comparing the contributions to the apparent free energy of membrane insertion of the 20 natural amino acids between the S. cerevisiae and the mammalian ER, we find that the two scales are strongly correlated but that the absolute difference between the most hydrophobic and most hydrophilic residues is approximately 2-fold smaller in S. cerevisiae.