Seeds development and germination processes is mainly affected by proteolysis process. Proteolysis is a metabolic pathway that involves comprehensive metabolic networks, different subcellular partitions and types of proteases, mostly cysteine-, serine-, aspartic- and metallo-proteases (van der Hoorn, 2008). Among the approximately 800 proteases encoded by plant genomes, more than 140 correspond to cysteine proteases (Rawlings et al., 2010). The main physiological role of cysteine proteases is metabolic degradation of peptides and proteins (Storer and Menard, 1994).
Several cysteine protease reported to play an important role in plant defense response against fungus (Höwing et al., 2014), viruz (Adenot et al., 2006), and bacteria (Bernoux et al., 2008). Papain-like cysteine proteinase genes also involved in a process of seed germination by hydrolysing seed storage proteins (Tsuji et al., 2013). Despite important role of cysteine endopeptidases, their uncontrolled activity can induce pathology and the destruction of tissue structures (Szewi?ska et al., 2016). In order to prevent such damage from excessive proteolysis activity, the regulatory mechanism has been developed by many organisms at the level of modified transcription, translation and post-translation synthesized by cells, in the form of specific protease inhibitors (Arai et al., 2002). In this role, cystatins are known as specific and natural inhibitors of papain-like cysteine proteases C1A family act as regulators of harmful cysteine proteases activities (Viswanathan et al. 2011). Composed with Leu-Ala-Arg-Phe-Ala-Val sequence motif, plant cystatins (phytocystatins) probably allows to recognize and regulate endogenous plant cysteine protease activities in the specific parts of plant tissues. (Abe et al., 1987; Callis, 1995; Volpicella et al., 2011). They were also reported to be involved in programmed cell death by modulating cysteine protease activity in the regulation of protein turnover (Solomon et al., 1999; Xu and Chye, 1999).
Numerous studies have been carried out extensively to explain the cystatins inhibitory ability and mechanism toward cysteine protease as its specific inhibitor. The results have shown that the inhibitor binds in a one-step process that is simple, reversible, and second-order type. In addition, those studies have revealed that enzymes with a blocked active centre could still bind cystatins, although with lower affinity (Bode et al., 1990). This indicates that cysteine protease–cystatin interactions are not based on a simple reaction with the catalytic cysteine residue of the enzyme, as is typical of substrates, but that they consist of hydrophobic contacts between the binding regions of cystatins and the corresponding residues forming the binding pockets of the enzyme. Despite their structural homology and similar mode of inhibition, cystatins display quite different enzyme affinities (Abrahamson, 1994).
In this study, Sesamum indicum cysteine protease (SiCP) have been molecularly characterized and comparative sequence alignment have analysed related with functional motifs. As plant cysteine protease was important biocatalysts in certain physiological functions of seedling growth and has demosntrated extensive functional diversity throughout various fields and industries, the recombinant protein was overexpressed and purified throughout prokaryotic expression system. The possible affinities interaction of cystatins in the control of SiCP activity during sesame seed germination is discussed.