Regulation of acidic ribosomal protein expression and phosphorylation in maize

Acidic ribosomal proteins (ARPs) are highly conserved phosphoproteins in eukaryotic organisms. They participate in translation regulation by interacting with eEF-2 elongation factors in the peptide elongation process. During maize germination, protein synthesis is tightly regulated by different mechanisms that are not yet clearly understood. The objective of this research is to characterize the expression patterns of the two maize ARPs (P1 and P2) and their phosphorylated status in germinating maize embryonic axes. Expression of P1 and P2 mRNA transcripts was analyzed by Northern blots with specific cDNA probes. Results indicated that both transcripts are among the mRNA stored pool of the quiescent axes and each displays a distinctive expression pattern during germination. P1 and P2 synthesis initiates very early in germination, as demonstrated by [(35)S]methionine pulse-labeling experiments. This synthesis was not insulin/IGF-stimulated as the synthesis of the bulk of ribosomal proteins that was responsive to this stimulus. P1 and P2 proteins were purified from ribosomes of maize embryonic axes and their physicochemical characteristics determined. A cytoplasmic pool of dephosphorylated P1 and P2 proteins was found in axes of quiescent and germinated stages that freely assembled into the ribosomes. IEF analysis of ARPs revealed one P1 (P1-1) and two P2 (P2-1 and P2-2) forms in the ribosomes of 24 h germinated axes. Kinetic studies of ARP phosphorylation during germination revealed a specific order of phospho-ARP appearance, suggesting that this process is under regulation within this period. It is concluded that P1 and P2 phosphorylation rather than ARP expression or assembly into ribosomes is the main step that regulates ARP function in axes during maize germination.