Serpins in plants and green algae

Abstract

Control of proteolysis is important for plant growth, development, responses to stress, and defence against insects and pathogens. Members of the serpin protein family are likely to play a critical role in this control through irreversible inhibition of endogenous and exogenous target proteinases. Serpins have been found in diverse species of the plant kingdom and represent a distinct clade among serpins in multicellular organisms. Serpins are also found in green algae, but the evolutionary relationship between these serpins and those of plants remains unknown. Plant serpins are potent inhibitors of mammalian serine proteinases of the chymotrypsin family in vitro but, intriguingly, plants and green algae lack endogenous members of this proteinase family, the most common targets for animal serpins. An Arabidopsis serpin with a conserved reactive centre is now known to be capable of inhibiting an endogenous cysteine proteinase. Here, knowledge of plant serpins in terms of sequence diversity, inhibitory specificity, gene expression and function is reviewed. This was advanced through a phylogenetic analysis of amino acid sequences of expressed plant serpins, delineation of plant serpin gene structures and prediction of inhibitory specificities based on identification of reactive centres. The review is intended to encourage elucidation of plant serpin functions.

A Review: The Role of Barley Seed Pathogenesis-Related Proteins (PRs) in Beer Production

“…Serpins were not originally classified as PRs. For their probable defensive role, hypothesized on the basis of irreversible inhibition of exogenous proteinases that break down seed storage proteins 72,132,133 , cereal serpins have been considered as PRs 111 . Protein Z accounts for 2% of the total protein content of malt and up to 25% of the non-dialyzable beer protein estimated by the enzyme-linked immunosorbant assay (ELISA) 36,37 .…”

A Review: Biological and Technological Functions of Barley Seed Pathogenesis-Related Proteins (PRs)

“…PIs can be divided into two classes, based on the protease inhibition mechanism: inhibitors employing standard mechanisms (Mm up to 22 kDa) and the serpins (serine proteinase inhibitors) (Mm ∼ 40 kDa) employing a so-called suicidal mechanism 281,282 . Standard inhibitors form a complex in which the reactive peptide bond is slowly hydrolyzed and the enzyme is liberated, but the rate of complex formation is much lower.…”

“…The majority of serpins are metastable proteins, able to inhibit serine proteinases of the chymotrypsin family by employing a unique "suicide substrate" mechanism of inhibition, different from standard mechanisms such as those of the Bowman-Birk or Kunitz families 281 . Cleavage of the reactive bond, in the reactive center of the protein during inhibition, is followed by a drastic conformational change and the subsequent formation of an enzyme-inhibitor complex (Fig.…”

“…The high content of lysine and gene expression in serpins regulated by the "high-Lys" alleles lys1 and lys3a 30 , as well as the pattern of accumulation and relative abundance during grain filling 90 , suggested their role as storage proteins in plants. Functions in defense have been suggested for cereal seed serpins 124,125,246,281 . Serpins are inactivated by low pH and thus not able to act as in vivo inhibitors of digestive proteinases in animals with acidic stomachs, but may protect storage proteins from insects' digestive serine proteinases of the chymotrypsin family in neutral or alkaline gut pH, or promote seed dispersal by seed-eating birds.…”

“…Due to high concentration in cereal endosperm and presence in the aqueous phase of bread dough, i.e., the dough liquor 291 , serpins have the potential to influence grain quality traits. Moreover, the amino acid sequence motifs of serpins resemble the Glnrich repeat sequences in prolamins, and formation of intermolecular disulfide bridges between serpins and between serpins and β-amylase has been observed 281 . The effects of 2 alleles (wild-type and null) of serpin, located on chromosome 5B, on the grain quality parameters such as dough extensibility and development time, flour water absorption, and milling yield has been studied 40 .…”