An In Vivo Investigation of Amino Acid Residues Critical for the Lectin Function of Arabidopsis Calreticulin 3

摘要

Calreticulin (CRT) is an endoplasmic reticulum (ER)-localized chaperone-like lectin that plays a crucial role in promoting the folding and maturation of newly synthesized glycoproteins and retaining incompletely/mis-folded proteins in the ER through its specific binding to monoglucosylated asparigine-linked glycans (GlcMan9GlcNAc2 with Glc, Man, and GlcNAc indicating glucose, mannose, and N-acetylglucosamine, respectively) (Michalak et al., 2009). CRT contains a cleavable signal peptide at its N-terminus and a tetrapeptide HDEL (His-Asp-Glu-Leu) ER retention/retrieval signal at its C-terminus. CRT3 and its ER membrane-anchored homolog calnexin (CNX) share a three-domain structure composed of a sugar-binding (3-sandwich globular N-domain, an extendedcentral proline-rich P-domain, and an acidic C-domain (Michalak et al., 2009). In vitro biochemical studies coupled with crystallographic analysis of recombinant CRT proteins have identified amino acid residues in the globular N-domain critical for itslectin function (Thomson and Williams, 2005; Gopalakrishnapai et al., 2006; Kozlov et al., 2010). However, no study verifying these in vitro findings in a living organism has been reported so far.