Characterization of Picg5 novel proteins associated with seasonal cold acclimation of white spruce (Picea glauca)

Abstract

Four protein bands of 38, 34, 32 and 25 kDa (designated as Picg5a to Picg5d) were detected specifically with an antibody against a putative PR10 protein Picg1 in Picea glauca. The Picg5 proteins accumulated during cold acclimation in foliage and roots and showed polymorphism in the population. Only one Picg5 protein was expressed in each individual seedling. The Picg5 protein levels were positively correlated with freezing tolerance (R2=0.6, P<0.001). Two types of cDNA clones (designated as Picg5-1 and Picg5-2) were characterized after screening an expressed cDNA library with anti-Picg1 antibody. Compared to Picg5-1, Picg5-2 has a deletion of 123 nucleotides in the coding region. Picg5-1 and Picg5-2 shared 97–98% identity on the nucleotide and deduced amino acid sequence levels. Southern blot analysis suggested one or two copies of the Picg5 gene within the P. glauca genome. Recombinant proteins by expressing Picg5 cDNAs in Escherichia coli had the same antigenic binding property and molecular masses of natural proteins, suggesting that they encode Picg5 proteins. The amino acid sequence deduced from Picg5 cDNAs consisted of five or six copy repeat segments (termed QKA segments). A BLASTP search of GenBank data revealed that the Picg5 proteins were novel ones, only two QKA segments showing low similarity with the K-segment of both dehydrin and rehydrin. Like protein accumulation, the Picg5 transcripts were up-regulated during cold acclimation. The expression pattern and unique features of protein structure suggest that the Picg5 proteins may have a function related to cold stress.