EARLI1 proteins localize to the cell wall and protect cells from freezing damage

Plants possess a large family of proteins sharing a conserved eight-cysteine-domain motif (8CM). Members of the 8CM family play important roles in plant growth, reproduction, and responses to both abiotic and biotic stresses. However, the biochemical functions of these proteins are largely unknown. Therefore, as part of my thesis I decided to biochemically analyze the 8CM-containing, hybrid proline-rich protein (HyPRP) EARLI1. EARLI1 is a cold responsive Arabidopsis gene that encodes a protein with a two-domain architecture: a proline-rich domain (PRD) at the N-terminus, and an 8CM domain at the C-terminus. I found that knocking down EARLI1 increased freezing-induced cellular damage. EARLI1-GFP fusion protein and immunocytochemical studies showed that the protein may be localized to the plant cell wall. Biochemical analyses showed that the PRD is hydrophilic, and that the 8CM domain is either hydrophobic or prone to aggregate in solution. Both domains have cysteine residues and produce higher-ordered complexes via disulfide bond formation. Higher ordered protein complexes recognized by anti-EARLI1 antiserum were also observed predominantly, but not exclusively, in cold acclimated plants. I propose that EARLI1 has a bimodular architecture in which the PRD interacts with the cell wall, and the 8CM domain with the plasma membrane. Stress induced bimodular proteins such as EARLI1 may play a role in protecting both the cell wall and the cell membrane when challenged by certain types of environmental stresses, including cold and possibly pathogen attack