C-terminal extension of calmodulin-like 3 protein from <italic>Oryza sativa</italic> L.: interaction with a high mobility group target protein

A large number of calmodulin-like (CML) proteins are present in plants, but there is little detailed information on the functions of these proteins in rice (Oryza sativa L.). Here, the CML3 protein from rice (OsCML3) and its truncated form lacking the C-terminal extension (OsCML3m) were found to exhibit a Ca2+-binding property and subsequent conformational change, but the ability to bind the CaM kinase II peptide was only observed for OsCML3m. Changes in their secondary structure upon Ca2+-binding measured by circular dichroism revealed that OsCML3m had a higher helical content than OsCML3. Moreover, OsCML3 was mainly localized in the plasma membrane, whereas OsCML3m was found in the nucleus. The rice high mobility group B1 (OsHMGB1) protein was identified as one of the putative OsCML3 target proteins. Bimolecular fluorescence complementation analysis revealed that OsHMGB1 bound OsCML3, OsCML3m or OsCML3s (cysteine to serine mutation at the prenylation site) in the nucleus presumably through the methionine and phenylalanine-rich hydrophobic patches, confirming that OsHMGB1 is a target protein in planta. The effect of OsCML3 or OsCML3m on the DNA-binding ability of OsHMGB1 was measured using an electrophoretic mobility shift assay. OsCML3m decreased the level of OsHMGB1 binding to pUC19 double-stranded DNA whereas OsCML3 did not. Taken together, OsCML3 probably provides a mechanism for manipulating the DNA-binding ability of OsHMGB1 in the nucleus and its C-terminal extension provides an intracellular Ca2+ regulatory switch.