The Solanaceous cystine-knot miniproteins are metallocarboxypeptidase inhibitors acting in stress defence and development.

The cystine-knot proteins identified in Solanaceous species are a group of small proteins (≤50 aa long in the mature form) characterized by a peculiar three dimensional structure called cystine-knot, which possess inhibitory activity against metallocarboxypeptidase. Their unique structural scaffold is due to the presence of three intertwined disulfide bonds conferring resistance to high T, extreme pH and proteolysis. Another characteristic of the cystine-knot miniproteins is the presence at the N terminus of a signal peptide for secretion. Studies on potato, tomato and tobacco cystine-knot metallocarboxypeptidase inihbitors (MCPI) demonstrated their responsiveness to various type of abiotic stresses. Moreover, the over-expression of a potato MCPI conferred resistance to the herbivor Chilo suppressalis and the fungus Magnaporthe oryzae in transgenic rice, suggesting a function in the plant response to biotic stress. More recently, it was shown that altering the expression pattern of two tomato cystine-knot metallocarboxypeptidase inhibitor (TCMPs) during flower and fruit development can result in early fruit production. To shed light on the role of these proteins in reproductive development, we have undertaken a yeast two hybrid analysis to detect interactive partners. One of the identified interactors belongs to the B-box zinc finger protein family which comprises Zn finger transcription factors with regulatory roles in different developmental processes and B-box proteins lacking the transcriptional regulation domain. TCMPs interact with a BBX protein of the second type whose principal function in A.thaliana is to interfere with the formation of protein complexes that regulate flowering. Our results indicate that the overexpression of TCMPs favours the sympodial termination in tomato and its ectopic overexpression in A. thaliana induces early flowering and increases FT expression, thus suggesting that TCMP might modulate the activity of flowering regulatory multiprotein complexes