2KXG

The solution structure of the squash aspartic acid proteinase inhibitor (SQAPI)


Experimental Data Snapshot

  • Method: SOLUTION NMR
  • Conformers Calculated: 100 
  • Conformers Submitted: 10 
  • Selection Criteria: structures with the lowest energy 

wwPDB Validation 3D Report Full Report


This is version 1.2 of the entry. See complete history

Literature

Solution structure of the squash aspartic acid proteinase inhibitor (SQAPI) and mutational analysis of pepsin inhibition.

Headey, S.J.Macaskill, U.K.Wright, M.A.Claridge, J.K.Edwards, P.J.B.Farley, P.C.Christeller, J.T.Laing, W.A.Pascal, S.M.

(2010) J.Biol.Chem. 285: 27019-27025

  • DOI: 10.1074/jbc.M110.137018

  • PubMed Abstract: 
  • The squash aspartic acid proteinase inhibitor (SQAPI), a proteinaceous proteinase inhibitor from squash, is an effective inhibitor of a range of aspartic proteinases. Proteinaceous aspartic proteinase inhibitors are rare in nature. The only other exa ...

    The squash aspartic acid proteinase inhibitor (SQAPI), a proteinaceous proteinase inhibitor from squash, is an effective inhibitor of a range of aspartic proteinases. Proteinaceous aspartic proteinase inhibitors are rare in nature. The only other example in plants probably evolved from a precursor serine proteinase inhibitor. Earlier work based on sequence homology modeling suggested SQAPI evolved from an ancestral cystatin. In this work, we determined the solution structure of SQAPI using NMR and show that SQAPI shares the same fold as a plant cystatin. The structure is characterized by a four-strand anti-parallel beta-sheet gripping an alpha-helix in an analogous manner to fingers of a hand gripping a tennis racquet. Truncation and site-specific mutagenesis revealed that the unstructured N terminus and the loop connecting beta-strands 1 and 2 are important for pepsin inhibition, but the loop connecting strands 3 and 4 is not. Using ambiguous restraints based on the mutagenesis results, SQAPI was then docked computationally to pepsin. The resulting model places the N-terminal strand of SQAPI in the S' side of the substrate binding cleft, whereas the first SQAPI loop binds on the S side of the cleft. The backbone of SQAPI does not interact with the pepsin catalytic Asp(32)-Asp(215) diad, thus avoiding cleavage. The data show that SQAPI does share homologous structural elements with cystatin and appears to retain a similar protease inhibitory mechanism despite its different target. This strongly supports our hypothesis that SQAPI evolved from an ancestral cystatin.


    Organizational Affiliation

    Institute of Fundamental Sciences, Massey University, Palmerston North 4442, New Zealand.




Macromolecules

Find similar proteins by: Sequence  |  Structure

Entity ID: 1
MoleculeChainsSequence LengthOrganismDetails
Aspartic protease inhibitor
A
95Cucurbita maximaN/A
Find proteins for Q6DLC9 (Cucurbita maxima)
Go to UniProtKB:  Q6DLC9
Experimental Data & Validation

Experimental Data

  • Method: SOLUTION NMR
  • Conformers Calculated: 100 
  • Conformers Submitted: 10 
  • Selection Criteria: structures with the lowest energy 
  • Olderado: 2KXG Olderado

Structure Validation

View Full Validation Report or Ramachandran Plots



Entry History 

Deposition Data

Revision History 

  • Version 1.0: 2010-06-09
    Type: Initial release
  • Version 1.1: 2011-07-13
    Type: Version format compliance
  • Version 1.2: 2013-01-30
    Type: Database references