The RPN1 Subunit of the 26S Proteasome in Arabidopsis Is Essential for Embryogenesis

Abstract

The 26S proteasome plays a central role in the degradation of regulatory proteins involved in a variety of developmental processes. It consists of two multisubunit protein complexes: the proteolytic core protease and the regulatory particle (RP). The function of most RP subunits is poorly understood. Here, we describe mutants in the Arabidopsis thaliana RPN1 subunit, which is encoded by two paralogous genes, RPN1a and RPN1b. Disruption of RPN1a caused embryo lethality, while RPN1b mutants showed no obvious abnormal phenotype. Embryos homozygous for rpn1a arrested at the globular stage with defects in the formation of the embryonic root, the protoderm, and procambium. Cyclin B1 protein was not degraded in these embryos, consistent with cell division defects. Double mutant plants (rpn1a/RPN1a rpn1b/rpn1b) produced embryos with a phenotype indistinguishable from that of the rpn1a single mutant. Thus, despite their largely overlapping expression patterns in flowers and developing seeds, the two isoforms do not share redundant functions during gametogenesis and embryogenesis. However, complementation of the rpn1a mutation with the coding region of RPN1b expressed under the control of the RPN1a promoter indicates that the two RPN1 isoforms are functionally equivalent. Overall, our data indicate that RPN1 activity is essential during embryogenesis, where it might participate in the destruction of a specific set of protein substrates.

Protein Abundance Changes and Ubiquitylation Targets Identified after Inhibition of the Proteasome with Syringolin A

“…CyclinB1;1 accumulates at the late G2/M phase of the cell cycle and at the end of the M phase it is degraded by the proteasome (46 -47). Inhibition or dysfunction of the proteasome can therefore be monitored using GUS staining in dividing cells (43,48). In expanding leaves, most cells have ceased cell division and grow by expansion except for meristemoid and procambial cells, the stem cells that continue to produce cells of the stomatal cell lineage in the epidermis and the vascular tissue in the mesophyll cell layer, respectively (47, 49 -50).…”

Dual Targeting of Arabidopsis HOLOCARBOXYLASE SYNTHETASE1: A Small Upstream Open Reading Frame Regulates Translation Initiation and Protein Targeting

“…Examples have been described in which genes do not act redundantly in vivo because of a difference of expression level or pattern. Such a scenario has been observed, for instance, for the proteasome subunit RPN1A and RPN1B: RPN1A lossof-function results in embryo lethality, whereas rpn1b mutants are fully viable and resemble the wild type (Brukhin et al, 2005). The nondispensable allele RPN1A is expressed at much higher levels than RPN1B.…”

Arabidopsis GLUTAMINE-RICH PROTEIN23Is Essential for Early Embryogenesis and Encodes a Novel Nuclear PPR Motif Protein That Interacts with RNA Polymerase II Subunit III

“…This is a critical stage at which the transition from pattern establishment to its refinement occurs. Examples of mutants arrested very early during embryogenesis include fac1, a zygotic embryo-lethal mutant (Xu et al, 2005), rpn1a, a mutation in the RNP1 subunit of the 26S proteasome (Brukhin et al, 2005), titan mutants, in which genes proposed to be involved in chromosome maintenance and microtubule assembly are disrupted (Liu and Meinke, 1998;McElver et al, 2000;Tzafrir et al, 2002), and opt3, a mutation of a peptide transporter gene (Stacey et al, 2002). Interestingly, among the predicted EMB genes there are 17 putative pentatricopeptide repeat (PPR)-containing genes, of which 7 were shown to be essential for late embryogenesis (Tzafrir et al, 2004;Cushing et al, 2005).…”