Molecular characterization and expression of an alfalfa protein with sequence similarity to mammalian ERp72, a glucose-regulated endoplasmic reticulum protein containing active site sequences of protein disulphide isomerase.

A complementary DNA clone (G1) containing sequence similarity to the mammalian lumenal endoplasmic reticulum protein ERp72 was isolated from an alfalfa (Medicago sativa L.) cDNA library by screening with a cDNA encoding human protein disulphide isomerase (PDI), which contains two thioredoxin-like active site regions which are highly conserved in ERp72. The polypeptide encoded by G1 consists of 364 amino acids, possesses a putative N-terminal secretory signal sequence and two regions, 113 amino acids apart, identical to the active sites of PDI and ERp72. G1 appears to be encoded by a small gene family in alfalfa, whose transcripts are constitutively expressed in all major organs of the plant. In alfalfa cell suspension cultures, G1 transcripts were markedly induced by treatment with tunicamycin, but not in response to calcium ionophore, heat shock or fungal elicitor. A similar expression pattern was observed for transcripts encoded by B2, a recently cloned alfalfa cDNA with strong sequence similarity to PDI. We discuss potential roles of plant proteins resembling vertebrate PDI and ERp72.