Proteins destined for the mitochondrial matrix are imported by the translocase of the outer membrane-the TOM complex-and the presequence translocase of the inner membrane-the TIM23 complex. At present, there is no structural information on components of the presequence translocase. Tim21, a subunit of the presequence translocase consisting of a membrane anchor and a carboxy-terminal domain exposed to the intermembrane space, directly connects the TOM and TIM23 complexes by binding to the intermembrane space domain of the Tom22 receptor. We crystallized the binding domain of Tim21 of Saccharomyces cerevisiae and determined its structure at 1.6 Å resolution. The Tim21 structure represents a new a/b-mixed protein fold with two a-helices flanked by an extended eight-stranded b-sheet. We also identified a core sequence of Tom22 that binds to Tim21. Furthermore, negatively charged amino-acid residues of Tom22 are important for binding to Tim21. Here we suggest a mechanism for the TOM-TIM interaction.

Spot the difference: Conformational analysis of the 2S,4R and 2S,4S epimers of N‐acetyl‐4‐mercaptopyrrolidine‐2‐carboxylic acid methyl esters reveals ring‐pucker preferences that are opposite of those of the hydroxyproline derivatives (see scheme). Replacement of proline or hydroxyproline in polypeptides with the chalcogen analogue should allow for fine‐tuning of the complex interplay of noncovalent interactions, steric hindrance, and stereoelectronic effects.

Collagens consist of three identical or different polypeptide chains which are arranged in a triple-helical supercoil of variable stability. Although the overall shape is that of a soft rod, collagens also contain stretches of low triple-helix stability and even disruptions of this structure. The local stability and recognition sites for interaction with other proteins of the extracellular matrix are defined and highly fine-tuned by the amino acids in the X and Y positions of the repeating Gly-Xaa-Yaa triplets with a Gly residue mandatory at every third position for the compact intertwining of three left-handed poly-Pro-II helices into the right-handed triple helix.[1] Extensive studies on synthetic model peptides have clearly identified the tripeptide unit Gly-Pro-Hyp as the most favorable triplet for stabilizing the triple helix, [1b, 2] and crystal structures of collagen peptides allowed detailed insights into the hydrogen-bonding networks of this very repetitive and regular tertiary structure. [3] From folding studies of natural collagens and related fragments a zipper-like folding mechanism was derived with the cis-to-trans isomerization of proline as the rate-limiting step. [1d, 4] In absence of cis-proline isomers the folding is very fast and completed within time scales comparable to the folding rates of other repetitive structures, such as the ahelix.[5] However, to increase our understanding of the folding and stability of the collagen triple helix, a system should be available that is equipped with an ultrafast conformational trigger which allows the induced folding/unfolding events to be monitored by time-resolved spectroscopy. This challenging task was realized in the present study by using a purposely designed azobenzene derivative as a photoswitchable conformational clamp which, when incorporated into the single collagen chains as side-chain-to-side-chain crosslink, provides the required changes in the conformational space to trigger folding/unfolding of the collagen triple helix.In preceding studies from our and other laboratories the concept of using azobenzene for ultrafast photomodulation of conformational states and thus of related biophysical properties has been validated.[6] The successful application of this principle for the photocontrol of a-helices [7] and recently of bhairpins [8] as the most simple tertiary structure motifs, led us to exploit the favorable optical properties of azobenzene, together with the regular supramolecular structure of the collagen triple helix, for the design of a model system suited for time-resolved spectroscopic studies on collagen folding and unfolding. The strategy applied was to crosslink two side chains of a collagen (Pro-Hyp-Gly) n peptide by the azobenzene chromophore (Scheme 1). As the collagen peptide an Nacetylated and C-amidated (Gly-Pro-Hyp) 7 -Gly-Gly was selected because of the relatively high thermal stability of its triple helix.[9] Molecular modeling served to identify suitable sequence positions in this peptide for grafting the azobenze...

Strukturen aufgedröselt: Die beiden Seitenketten eines Collagenpeptids mit (2S,4S)‐Mercaptoprolin an zwei definierten Positionen wurden durch ein Diiodazobenzol als Verbindungsstück verknüpft. Das Peptid faltet mit der trans‐konfigurierten Azobenzolklammer (orange) in eine Tripelhelix (grün, blau und grau) und entfaltet bei Belichtung bei 330 nm. Die lichtgesteuerten Faltungs‐ und Entfaltungsvorgänge sind völlig reversibel, sodass dieses System für ultraschnelle Spektroskopie prädestiniert ist.