Nutrient limitation alters the levels of two distinct forms of cyclin-dependent kinase G2, a putative autophagy progression modulator in tobacco BY-2 cells

By screening protein kinases that retard autophagy progression in tobacco (Nicotiana tabacum) BY-2 cells, we found that a tobacco cyclin-dependent kinase G2 (NtCDKG2) would be involved in autophagy progression under sucrose-starvation condition. Characterization of its cDNA, and comparison with corresponding mRNA and genome sequences in parental species of tobacco indicated that the NtCDKG2 gene originated from N. tomentosiformis. Comparison of the deduced amino acid sequences of NtCDKG2 cDNA with those of N. tomentosiformis indicated that the protein encoded by the cDNA (NtCDKG2;1) is identical to a shorter variant of the CDKG2 protein. Characterization of NtCDKG2;1 protein using specific antibodies indicated that 73 kDa and 48 kDa forms, designated as NtCDKG2;1(p73) and NtCDKG2;1(p48), present in tobacco BY-2 cells. NtCDKG2;1(p73) decreased under starvation of either sucrose or phosphate whereas NtCDKG2;1(p48) decreased under starvation of sucrose and nitrogen. These data suggest that the roles of these two forms differ in tobacco cells.