The squash aspartic proteinase inhibitor SQAPI is widely present in the cucurbitales, comprises a small multigene family, and is a member of the phytocystatin family.

Full Abstract

The squash (Cucurbita maxima) phloem exudate-expressed aspartic proteinase inhibitor (SQAPI) is a novel aspartic acid proteinase inhibitor, constituting a fifth family of aspartic proteinase inhibitors. However, a comparison of the SQAPI sequence to the phytocystatin (a cysteine proteinase inhibitor) family sequences showed approximately 30% identity. Modeling SQAPI onto the structure of oryzacystatin gave an excellent fit; regions identified as proteinase binding loops in cystatin coincided with regions of SQAPI identified as hypervariable, and tryptophan fluorescence changes were also consistent with a cystatin structure. We show that SQAPI exists as a small gene family. Characterization of mRNA and clone walking of genomic DNA (gDNA) produced 10 different but highly homologous SQAPI genes from Cucurbita maxima and the small family size was confirmed by Southern blotting, where evidence for at least five loci was obtained. Using primers designed from squash sequences, PCR of gDNA showed the presence of SQAPI genes in other members of the Cucurbitaceae and in representative members of Coriariaceae, Corynocarpaceae, and Begoniaceae. Thus, at least four of seven families of the order Cucurbitales possess member species with SQAPI genes, covering approximately 99% of the species in this order. A phylogenetic analysis of these Cucurbitales SQAPI genes indicated not only that SQAPI was present in the Cucurbitales ancestor but also that gene duplication has occurred during evolution of the order. Phytocystatins are widespread throughout the plant kingdom, suggesting that SQAPI has evolved recently from a phytocystatin ancestor. This appears to be the first instance of a cystatin being recruited as a proteinase inhibitor of another proteinase family.

Learn Faster Today Improve your study skills

Author information

Author/s: Christeller, John T (JT); Farley, Peter C (PC); Marshall, Richelle K (RK); Anandan, Ananda (A); Wright, Michele M (MM); Newcomb, Richard D (RD); Laing, William A (WA);

Affiliation: Horticulture and Food Research Institute, Private Bag 11030, Palmerston North, New Zealand. jchristeller(-atsign-)hortresearch.co.nz

Journal and publication information

Publication Type: Journal Article; Research Support, Non-U.S. Gov't

Journal: Journal of molecular evolution (J Mol Evol), published in Germany. (Language: eng)

Reference: 2006-Dec; vol 63 (issue 6) : pp 747-57

Dates: Created 2006/12/05; Completed 2007/03/29;

PMID: 17103059, status: MEDLINE (last retrieval date: 12/26/2008)

Sourced from the National Library of Medicine. Abstract text and other information may be subject to copyright.

External Links for this article (including full text providers, if available):

Click Electronic Full-text Provider Links to see options for finding the electronic full text links to this article. Note there may be a subscription or fee required for access to the full text. See our FAQ for information on finding FREE full text articles.

This article may also be located in paper journal collections available in many libraries. Use the Journal and Publication Information above to find the full article.

MeSH headings (categories)

This article was linked to the MESH Headings shown below.

Amino Acid Sequence
Aspartic Endopeptidases - antagonists & inhibitors
Cucurbitaceae - genetics
Cystatins - genetics
Genome, Plant

Associated Chemicals: Cystatins (0) ; Plant Proteins (0) ; SQAPI protein, Cucurbita maxima (0) ; Aspartic Endopeptidases (EC 3.4.23.-)

These are the highest related articles currently in the database:

See 100+ related articles.