Biomedical subjects

Martin Sagermann

Publications and source records attributed to Martin Sagermann.

5 recordsLinked to original sources

PubMed2006

Sequential reorganization of beta-sheet topology by insertion of a single strand.

Insertions, duplications, and deletions of sequence segments are thought to be major evolutionary mechanisms that increase the structural and functional diversity of proteins. Alternative splicing, for example, is an intracellular editing mechanism that is thought to generate isoforms for 30%-50% of all human genes. Whereas the inserted sequences usually display only minor structural rearrangements at the insertion site, recent observations indicate that they may also cause more dramatic structural displacements of adjacent structures. In the present study we test how artificially inserted sequences change the structure of the beta-sheet region in T4 lysozyme. Copies of two different beta-strands were inserted into two different loops of the beta-sheet, and the structures were determined. Not surprisingly, one insert "loops out" at its insertion site and forms a new small beta-hairpin structure. Unexpectedly, however, the second insertion leads to displacement of adjacent strands and a sequential reorganization of the beta-sheet topology. Even though the insertions were performed at two different sites, looping out occurred at the C-terminal end of the same beta-strand. Reasons as to why a non-native sequence would be recruited to replace that which occurs in the native protein are discussed. Our results illustrate how sequence insertions can facilitate protein evolution through both local and nonlocal changes in structure.

Amino Acid Motifs↗

PubMed2004

Relocation or duplication of the helix A sequence of T4 lysozyme causes only modest changes in structure but can increase or decrease the rate of folding.

In T4 lysozyme, helix A is located at the amino terminus of the sequence but is associated with the C-terminal domain in the folded structure. To investigate the implications of this arrangement for the folding of the protein, we first created a circularly permuted variant with a new amino terminus at residue 12. In effect, this moves the sequence corresponding to helix A from the N- to the C-terminus of the molecule. The protein crystallized nonisomorphously with the wild type but has a very similar structure, showing that the unit consisting of helix A and the C-terminal domain can be reconstituted from a contiguous polypeptide chain. The protein is less stable than the wild type but folds slightly faster. We then produced a second variant in which the helix A sequence was appended at the C-terminus (as in the first variant), but was also restored at the N-terminus (as in the wild type). This variant has two helix A sequences, one at the N-terminus and the other at the C-terminus, each of which can compete for the same site in the folded protein. The crystal structure shows that it is the N-terminal sequence that folds in a manner similar to that of the wild type, whereas the copy at the C-terminus is forced to loop out. The stability of this protein is much closer to that of the wild type, but its rate of folding is significantly slower. The reduction in rate is attributed to the presence of the two identical sequence segments which compete for a single, mutually exclusive, site.

Bacteriophage T4↗

PubMed2003

Long-distance conformational changes in a protein engineered by modulated sequence duplication.

There are few, if any, known instances in which a biological signal is transmitted via a large conformational change through the body of a protein. We describe here a mutant of T4 lysozyme that was engineered to permit structural change at a distance. The design uses a tandem sequence repeat that makes it possible to transmit large-scale structural changes from one end of an alpha-helix to the other over a distance of 17-25 A. The method should be of general applicability and may make it possible to introduce a mutation at one site in a protein that will induce large-scale changes in the structure at a spatially remote site.

Binding Sites↗

PubMed2002

Crystal structures of a T4-lysozyme duplication-extension mutant demonstrate that the highly conserved beta-sheet region has low intrinsic folding propensity.

Residues 24 to 35 of T4 lysozyme correspond to the second and third strands of a region of beta-sheet that is highly conserved in all known lysozyme and chitinase structures. To evaluate the intrinsic propensity of these amino acid residues to form a defined structure they were added at the C terminus of the native protein, together with a dipeptide linker. Two crystal structures of this active, mutant protein were obtained, to 1.9A and 2.3A resolution, respectively. Even though the crystal conditions are similar, the appended sequence adopts very different secondary structures. In one case it is weakly structured and appears to extend through the active-site cleft, perhaps in part adding an extra strand to the original beta-sheet. In the other crystal form the extension is largely alpha-helical. The formation of these alternative structures shows that the sequence does not have a strong intrinsic propensity to form a unique fold (either beta-sheet or otherwise). The results also suggest that structural conservation during evolution does not necessarily depend on sequence conservation or the conservation of folding propensity.

Amino Acid Sequence↗

PubMed2002

A test of proposed rules for helix capping: implications for protein design.

alpha-helices within proteins are often terminated (capped) by distinctive configurations of the polypeptide chain. Two common arrangements are the Schellman motif and the alternative alpha(L) motif. Rose and coworkers developed stereochemical rules to identify the locations of such motifs in proteins of unknown structure based only on their amino acid sequences. To check the effectiveness of these rules, they made specific predictions regarding the structural and thermodynamic consequences of certain mutations in T4 lysozyme. We have constructed these mutants and show here that they have neither the structure nor the stability that was predicted. The results show the complexity of the protein-folding problem. Comparison of known protein structures may show that a characteristic sequence of amino acids (a sequence motif) corresponds to a conserved structural motif. In any particular protein, however, changes in other parts of the sequence may result in a different conformation. The structure is determined by sequence as a whole, not by parts considered in isolation.

Amino Acid Motifs↗