Metalloprotein-Specific or Critical Amino Acid Residues: Perspectives on Plant-Precise Detoxification and Recognition Mechanisms under Cadmium Stress

Abstract

Cadmium (Cd) pollution in cultivated land is caused by irresistible geological factors and human activities; intense diffusion and migration have seriously affected the safety of food crops. Plants have evolved mechanisms to control excessive influx of Cd in the environment, such as directional transport, chelation and detoxification. This is done by some specific metalloproteins, whose key amino acid motifs have been investigated by scientists one by one. The application of powerful cell biology, crystal structure science, and molecular probe targeted labeling technology has identified a series of protein families involved in the influx, transport and detoxification of the heavy metal Cd. This review summarizes them as influx proteins (NRAMP, ZIP), chelating proteins (MT, PDF), vacuolar proteins (CAX, ABCC, MTP), long-distance transport proteins (OPT, HMA) and efflux proteins (PCR, ABCG). We selected representative proteins from each family, and compared their amino acid sequence, motif structure, subcellular location, tissue specific distribution and other characteristics of differences and common points, so as to summarize the key residues of the Cd binding target. Then, we explain its special mechanism of action from the molecular structure. In conclusion, this review is expected to provide a reference for the exploration of key amino acid targets of Cd, and lay a foundation for the intelligent design and breeding of crops with high/low Cd accumulation.

Keywords: Cd transport and detoxification; H/DXXXD; cysteine-related residues; functional protein.

Figure 1

Plant hormone signaling regulation and related gene expression analysis under Cd stress. A: The regulatory pathway of plant hormones under Cd stress. Plant hormones such as ABA, JA, SA enhance the detoxification ability of plants against cadmium stress by regulating the expression of downstream proteins. TFs stands for transcription factors. The arrows with solid lines indicate that the regulation methods have been reported so far, and the arrows with dotted lines indicate that the regulation methods are not yet clear. Polyline arrows indicate exogenous additions. B: Effects of ABA, JA and SA on the expression of genes involved in Cd transport and detoxification (taking Arabidopsis as an example). Expression data comes from the database (http://ipf.sustech.edu.cn/pub/athrna/) (accessed on 30 December 2021). A red box means more correlation, a white box means less correlation. Plant hormones like ABA, JA, SA are closely related to the expression of these genes.

Figure 2

Residue-specific and transmembrane structural characterization of proteins. In each family, 1–2 special proteins involved in Cd influx, long-distance transport, vacuolar transport, chelating, and efflux were selected. Each type of protein is marked with a different colored box. Their specific residues and transmembrane structure characteristics were analyzed. The remaining member information can be found in Figures S1–S10, and their amino acid sequences are also summarized in Table S1. Different residues are indicated by different colors. The amino acid composition of the residue is indicated by the abbreviated letter of the amino acid in the circle. Yellow rectangles represent biofilms, green boxes represent N-glycan motifs, and red circles and white font represent signal peptides.