Leucyl aminopeptidase

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Leucine aminopeptidase
Chr. 4 p15.33
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Leucyl aminopeptidases (

solanaceous-specific acidic LAP (LAP-A) in tomato
(Solanum lycopersicum).

Enzyme description, structure, and active site

LAP-A active site residues. Two Zn+2 cations are also shown, along with a water and a bicarbonate ion that acts as a general base.

The

hexamers in vivo. The Gu et al. from 1999 demonstrated that six 55kDA enzymatically inactive LAP-A protomers come together to form the 353kDa bioactive LAP-A hexamer. Structures of the bovine lens LAP protomer and the biologically active hexamer have been constructed[4]
can be found through Protein Data Bank (2J9A).

Mechanism(s)

Historically, the mechanisms of carboxypeptidases and endoprotease have been much more well-studied and understood by researchers (Ref #6 Lipscomb 1990). Work within the past two decades has provided vital knowledge regarding the mechanisms of aminopeptidases. The mechanism of

In this mechanism, the bicarbonate ion acts as a general base. For LAP-A, R1 could be the R group of leucine, methionine, or arginine.

bovine lens LAP and PepA have been elucidated (Ref 1 and 2), however, the exact mechanism of tomato LAP-A is unknown at this time. A search of current literature does not indicate that new research is underway to determine the exact mechanism of LAP-A. Based on the biochemical similarities of the LAPs between kingdoms, the mechanism of LAP-A may be similar to bovine lens LAP and PepA.

Biological function

Once thought of as a

immune
response in tomato.

Background on plant immune response

In order to survive, plants must be able to respond to many

genes whose expression changes over time. The early genes amplify the wounding signal and can be detected 30 minutes to 2 hours after damage (Ryan 2000). Late gene expression can be seen 4–24 hours after wounding. Products of late-response genes act as deterrents to chewing-insect feeding, often by decreasing the nutritional value of the food ingested or interfering with insect gut function (Walling 2000). For example, serine proteinase inhibitors (Pins) interfere with digestive proteases in the insect gut and polyphenol oxidases (PPO) act to decrease the nutritive value of plant leaves after ingestion by herbivores (Johnson et al. 1989; Ryan 2000; Orozco-Cardenas 2001). Please see the Picture 3 for a summary of the wound response in tomato.

The wounding response pathway as studied in tomato.

The plant response in this octadecanoid pathway is similar to

and leukotriene pathways (Ref Walling 2000). This particular pathway is inhibited by salicylic acid.

Octadecanoid pathway

(LAP-A), a product of the octadecanoid pathway in some solanaceous plants, has been shown by Fowler et al. to have a regulatory role in the late wound response of tomato. Experiments were conducted using three

wildtype (WT), (LapA-SI) plants that were silenced for LAP-A, and LapA-OX that constitutively expressed LAP-A. Late-gene expression was inhibited in wounded LapA-SI plants, and the LapA-SI plants were also more susceptible to tobacco hornworm feeding, relative to wildtype (WT) plants. In comparison, the wounded LapA-OX leaves exhibited heightened levels of late gene RNA accumulation, an increased resistance to herbivory, and extended expression of late wound-response genes. These data suggest that LAP-A functions in regulating both the intensity and the persistence of the late wound response. However, unwounded LapA-OX did not accumulate late gene RNA
transcripts, suggesting that presence of LAP-A alone is not sufficient to induce late gene expression. LAP-A is the first plant aminopeptidase shown to have a regulatory role in signal transduction pathway.

Osmoregulation

LAP proteins are expressed in a variety of marine organisms as a method of coping with the osmotic threat high salinity poses to the cell. During bouts of high salinity, LAP begins the catalysis of proteins in order to release amino acids into the cell in an attempt to balance the high ion concentrations in the external environment.[5]

References

Sources

External links